> **来源：[研报客](https://pc.yanbaoke.cn)** # Air quality indexes Key considerations and roadmaps for best practices # Air quality indexes # Key considerations and roadmaps for best practices # Abstract Air quality indexes (AQIs) are widely used to communicate short-term air pollution concentrations and related health risks to the public. Conventional AQIs are typically formulated based on the concentration of the single air pollutant that most exceeds its regulatory standard (among all pollutants measured). Alternatively, health-based AQIs represent the combined health risks due to multiple air pollutants and are formulated from concentration-response functions derived from epidemiological evidence. This report examines public health approaches to improve AQIs, with a focus on roadmaps for best practices in developing, validating and communicating health-based AQIs. It first reviews the status of and differences between conventional and health-based AQIs. The Canadian Air Quality Health Index is presented as a model health-based approach, and the strengths and weaknesses of several novel indexes are discussed. Next, published studies supporting conventional and health-based AQIs across several tiers of evaluation are reviewed, focusing on evidence of public health benefits. The roles of public communication and global equity are also discussed. From this assessment, key considerations that serve as the basis for framing roadmaps for best practices moving forward are identified. In conclusion, health-based AQIs are noted to offer several advantages compared with conventional AQIs. There is a need to adopt locally-adapted, equity-sensitive approaches that reflect diverse air pollution profiles, health susceptibilities and cultural contexts. Strengthening risk communication – through improved indexes and innovative strategies – plays a key role in supporting future efforts to protect public health. # Keywords AIR POLLUTION, AIR POLLUTANTS, ENVIRONMENTAL EXPOSURE, ENVIRONMENT AND PUBLIC HEALTH, HEALTH COMMUNICATION, PRIMARY PREVENTION ISBN: 9789289062701 (PDF) # © World Health Organization 2026 Some rights reserved. This work is available under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 IGO licence (CC BY-NC-SA 3.0 IGO; https://creativecommons.org/licenses/by-nc-sa/3.0/igo). Under the terms of this licence, you may copy, redistribute and adapt the work for non-commercial purposes, provided the work is appropriately cited, as indicated below. In any use of this work, there should be no suggestion that WHO endorses any specific organization, products or services. The use of the WHO logo is not permitted. 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CIP data are available at http://apps.who.int/iris. Sales, rights and licensing. To purchase WHO publications, see http://apps.who.int/bookorders. To submit requests for commercial use and queries on rights and licensing, see http://www.who.int/about/licensing. Third-party materials. If you wish to reuse material from this work that is attributed to a third party, such as tables, figures or images, it is your responsibility to determine whether permission is needed for that reuse and to obtain permission from the copyright holder. The risk of claims resulting from infringement of any third-party-owned component in the work rests solely with the user. General disclaimers. The designations employed and the presentation of the material in this publication do not imply the expression of any opinion whatsoever on the part of WHO concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. Dotted and dashed lines on maps represent approximate border lines for which there may not yet be full agreement. The mention of specific companies or of certain manufacturers' products does not imply that they are endorsed or recommended by WHO in preference to others of a similar nature that are not mentioned. Errors and omissions excepted, the names of proprietary products are distinguished by initial capital letters. All reasonable precautions have been taken by WHO to verify the information contained in this publication. However, the published material is being distributed without warranty of any kind, either expressed or implied. The responsibility for the interpretation and use of the material lies with the reader. In no event shall WHO be liable for damages arising from its use. Design: Imre Sebestyen, jr / Unit Graphics # Contents # Acknowledgements ii # Abbreviations iii # 1. Introduction 1.1 Background 1.2 Rationale and objectives 3 1.3 Methodology 4 # 2. Scientific review 5 2.1 Overview of AQIs 5 2.2 Existing and novel health-based AQIs 9 2.3 Evaluation of AQIs and alerts 16 2.4 Public communication of AQIs 27 2.5 Global equity 31 # 3. Key considerations and roadmaps for future AQ(H)Is 33 3.1 Development 33 3.2 Communication 39 3.3 Validation 40 3.4 Further refinement and global equity 42 # 4. Conclusions 45 # References 47 # Acknowledgements The WHO Regional Office for Europe thanks all experts for their contributions to developing this report. The report was prepared by Jeffrey R. Brook (University of Toronto, Canada), Robert D. Brook (Wayne State University, United States of America), Julia C. Fussell (Imperial College London, United Kingdom), Frank J. Kelly (Imperial College London, United Kingdom) and Sanjay Rajagopalan (Harrington Heart and Vascular Institute, University Hospitals and Case Western Reserve University, United States). Robert D. Brook provided overall scientific leadership throughout the development of the report. The report was coordinated and produced by the WHO European Centre for Environment and Health, WHO Regional Office for Europe. Román Pérez Velasco provided conceptual guidance, contributed written inputs, reviewed draft versions and oversaw the external review process. The work was carried out under the overall technical commenting and review of Dorota Jarosńska and Francesca Racioppi. The following external reviewers from various disciplines provided constructive comments on an earlier version of this report: Alberto González Ortiz (European Environment Agency, Denmark), Andrea Hinwood (United Nations Environment Programme, Kenya), Bin Jalaludin (University of New South Wales, Australia), Heli Lehtomäki (Finnish Institute for Health and Welfare, Finland), Ebba Malmqvist (Lund University, Sweden), Alexandra Monteiro (University of Aveiro, Portugal), A. Susana Ramírez (University of California, Merced), Rebecca K. Saari (University of Waterloo, Canada), David Stieb (formerly of Health Canada and University of Ottawa, Canada), Myriam Tobollik (German Environment Agency, Germany) and Janine Wichmann (University of Pretoria, South Africa). Thanks also go to Pierpaolo Mudu (World Health Organization) and Jiawei Zhang (University of Copenhagen, Denmark) for providing specific inputs. This report was produced with the financial assistance of the European Commission (Directorate-General for Environment) and the Federal Ministry for the Environment, Climate Action, Nature Conservation and Nuclear Safety (Germany). This publication was co-funded by the European Union. Its contents are the sole responsibility of the WHO Regional Office for Europe and do not necessarily reflect the views of the European Union. Co-funded by the European Union # Abbreviations AQG air quality guideline (levels) AQHI air quality health index AQL air quality index CO carbon monoxide CVD cardiovascular disease EU European Union LCS low-cost sensors $\mathrm{O}_{3}$ ozone $\mathrm{NO}_{2}$ nitrogen dioxide $\mathrm{PM}_{2.5}$ particulate matter of $\leq 2.5 \mu \mathrm{m}$ in aerodynamic diameter $\mathrm{PM}_{10}$ particulate matter of $\leq 10~\mu \mathrm{m}$ in aerodynamic diameter $\mathrm{SO}_2$ sulfur dioxide # 1. Introduction # 1.1 Background WHO estimates that globally 4.2 million deaths per year and a loss of 105.2 million disability-adjusted life years are attributable to ambient (outdoor) air pollution (principally derived from fossil fuel combustion) (1,2). These health figures reflect the impact of long-term exposure to fine particulate matter of $\leq 2.5\mu \mathrm{m}$ in aerodynamic diameter $(\mathsf{PM}_{2.5})$ on five diseases: acute lower respiratory infections, chronic obstructive pulmonary disease, ischaemic heart disease, stroke and lung cancer. As additional diseases (e.g. type 2 diabetes, neurological disorders) and pollutants (e.g. nitrogen dioxide $(\mathsf{NO}_2)$ and ozone $(\mathsf{O}_3))$ are not considered, the total global public health burden from all ambient air pollution is likely to be even larger (3,4). In 2021 WHO reduced most of its air quality guideline (AQG) levels in accordance with accruing data that show adverse health effects at low air pollutant concentrations with no observable thresholds below which long-term exposures can be considered safe, especially for $\mathsf{PM}_{2.5}$ (5). Under these updated criteria, $99\%$ of the global population live in areas where air pollution concentrations exceed annual WHO AQG levels (6,7). However, ambient air pollution levels vary substantially worldwide. Low- and middle-income countries face $\mathsf{PM}_{2.5}$ levels that are roughly threefold higher than in high-income countries (5,7,8). The WHO African, South-East Asia and Eastern Mediterranean regions are those most heavily impacted by poor air quality. While lowering trends in air pollution concentrations have been reported in many (but not all) locations, there is rising concern that climate change may hamper further improvements or even worsen air quality in the future (9). In addition to the immense public health threat posed by long-term exposure to air pollution, robust evidence shows that even short-term elevations (e.g. for 24 hours) in air pollution levels promote acute increases in morbidity and mortality independent of (and for all levels of) the chronic prevailing concentrations (10-12). Approximately 1 million deaths worldwide are attributable each year to acute (i.e. 24-hour) elevations in $\mathsf{PM}_{2.5}$ levels alone (12). In this context, government agencies in many countries have developed and regularly issue air quality indexes (AQIs) (Box 1) (13). Their aim is to communicate easy-to-understand information to the public on local short-term air quality, as well as the related health risks (14-16). Practical advice on behaviour changes for individuals is also often provided to help to reduce exposures. # Box 1. Report terminology # Air pollution indexes AQI: conventional indexes calculated from short-term individual air pollution concentrations with thresholds most commonly derived from national regulations. In general, the single air pollutant (among all pollutants measured) that most exceeds its regulatory standard sets the AQI level. Health-based AQI/air quality health index (AQHI): indexes of this type represent short-term health risks from (typically) multipollutant air pollution exposures derived from epidemiological evidence. This report uses the term AQHI synonymously with health-based AQI, except when referring specifically to the Canadian AQHI. Air quality alert: the active issuance of warnings to the public (e.g. via the internet, text messages on mobile networks) when AQIs, AQHIs or individual air pollution concentrations reach specified high thresholds. AQ(H)Is: encompasses both conventional AQIs and AQHIs. Raw air quality data: individual air pollutant concentrations, as released to the public. AQ(H)I values: numbers (i.e. unitless values) denoting the concentration of a specific air pollutant across a uniform scale (AQI) or the risk of a specific health outcome (AQHI). AQ(H)I categories or bands: a range of AQ(H)I values comprising air pollution exposures (AQIs) or health risks (AQHIs) for which the same behavioural advice is issued for subsets of the population. # Geographical scales of indexes Local: single communities or cities. Regional: counties, states, provinces and governmental regions. National: individual countries. Multicountry: a group of neighbouring countries, for example, the European Union (EU). # Populations Susceptible: individuals/populations who are prone to greater health risks than healthy adults at the same concentration of pollution exposures. Vulnerable: individuals/populations who face more frequent and/or higher exposures to air pollution. # Understanding and applying information Health literacy: the personal knowledge and competencies that accumulate through daily activities, social interactions and across generations. These are mediated by the organizational structures and availability of resources that enable people to access, understand, appraise and use information and services in ways that promote and maintain good health and well-being for themselves and those around them (17). Environmental health literacy: the basic knowledge and skills needed to comprehend environmental health risks and to devise, assess, implement and evaluate potential solutions (78). The level of environmental health literacy determines how individuals and communities make sense of and act on health-related information about environmental hazards (78). # 1.2 Rationale and objectives Effective communication on the health risks associated with air pollution is an important element of several WHO policy frameworks. World Health Assembly resolution WHA68.8 on health and the environment: addressing the health impacts from air pollution (79) and the accompanying roadmap (20) call for action to inform policy-makers and the public about the adverse health effects of air pollution and the benefits of implementing appropriate actions. Similarly, the WHO global strategy on health, environment and climate change identified the critical importance of enhanced communication, increased awareness and active public engagement (21). At European level, in 2017 the Ostrava Declaration of the Sixth Ministerial Conference on Environment and Health recognized the responsibility of authorities to improve public understanding of air quality issues through targeted communication strategies (22). Building on this, in 2023 the Budapest Declaration of the Seventh Ministerial Conference advocated strengthening communication tools and improving literacy regarding the interconnectedness of health, environment and climate change (23). This Declaration also highlighted the importance of expanding access to information and facilitating public participation in decision-making processes related to the environment and health. In May 2025 the Seventy-eighth World Health Assembly adopted the Updated road map for an enhanced global response to the adverse health effects of air pollution (24), which paves the way for global health action until 2030. The roadmap identified strategic communication as an important component to facilitate the integration of air pollution reduction efforts within broader health and development initiatives. Recognizing that community awareness and the dissemination of accurate, evidence-based information are fundamental to catalysing multisectoral action, the roadmap identified the health sector as a key agent in disseminating evidence on health risks and exposure reduction strategies. At global level, the promotion of and support for the right of access for all to information about pollution and pollutants is one of the intended outcomes of the joint efforts of The United Nations system: common approach towards a pollution-free planet (25). At EU level, Directive (EU) 2024/2881 on ambient air quality and cleaner air for Europe should be transposed into national laws by 2026. The Directive stipulates several requirements related to the provision of air quality information to the public. Each EU Member State is required to make publicly available a clear and easily understandable AQI that provides hourly updates for key pollutants. Wherever feasible, this AQI should be comparable across countries, follow WHO recommendations and be based on the European Air Quality Index (26,27), including health-related information for at-risk groups. Countries may use the European AQI directly (or develop their own), but are requested to reference it if they choose not to adopt it. A generic communication requirement involves sharing information on symptoms linked to high air pollution levels and on reducing personal exposure, especially in areas frequented by at-risk populations, such as health-care facilities (28). In response to a call by experts for WHO to promote health-based AQIs (29) and building on WHO's recent work to support evidence-based risk communication (5,15,29-31), including at the country level (32), the current report aims to provide a comprehensive overview of AQIs from a public health perspective. The report focuses on health-based AQIs or AQHIs, including a review of existing practices, salient issues and key considerations to strengthen their implementation. The main target audiences are authorities in charge of providing air quality information to the public, as well as technical experts, health-care professionals and researchers in the field of air quality. # 1.3 Methodology A structured approach was applied to guide the development of this report, combining a targeted review of the literature, critical appraisal by an expert writing group, and an external review process. The expert writing group had expertise in atmospheric science, clinical medicine, communication, environmental epidemiology, health policy, population health and toxicology. The evidence base was compiled through a PubMed search for peer-reviewed literature conducted in October 2024 using relevant search terms, supplemented by grey literature, including WHO reports and other key documents identified by the expert writing group. Relevant papers were summarized to capture current practices and key considerations, with the aim of informing a roadmap for best practices. Insights from the expert writing group were incorporated throughout the process through structured email exchanges and virtual meetings. A preliminary draft of the report underwent external review by experts and stakeholders, who contributed additional perspectives in areas such as environmental engineering and behavioural science. Their feedback was assessed and incorporated where appropriate. Throughout the report, the Canadian AQHI, the United Kingdom's daily AQI and the United States AQI were often highlighted as illustrative examples because they were (among) the first official indexes in their respective categories, have long records of real-world use and have more extensively informed research and development efforts. The report is organized into three further chapters: Chapter 2 provides a review of the science related to AQ(H)Is, Chapter 3 contains key considerations and roadmaps to enhance current practices, and Chapter 4 contains concluding remarks. # 2. Scientific review # 2.1 Overview of AQIs # 2.1.1 What is an AQI? The threat of ambient air pollution to human health (2,6,7) and evidence that reduced exposures yield observable health benefits (33,34) call for the provision of accessible and useful information to protect the public from poor air quality (5,29,30). For short-term fluctuations in air quality, one mechanism for achieving this includes AQIs. Official AQIs are regulatory-based communication and risk-awareness tools issued by government agencies in many countries on a frequent basis (15,35). Around the world, AQIs vary in relation to many factors, including thresholds (or breakpoints) for categories (or bands) owing to prevailing pollution concentrations, number of AQI categories and their pollution ranges, formulations for calculations, whether they are derived from single pollutants or mixtures, relevant geographical areas, and whether they are considered health based, such as the Canadian AQHI (Box 1) (13,16). As reviewed in a 2023 WHO Regional Office for Europe report, country-specific AQIs and public communication strategies differ substantially within the WHO European Region (15). All AQI types, however, communicate information to the public only about short-term local air quality and related health risks with the intention of influencing people's behaviour (over hours to days) and reducing pollution-related acute health risks (36). # 2.1.2 Construction of AQIs To be informative for the public, AQIs must translate complex air quality information into an understandable, clear and concise format. This is achieved by converting air pollution concentration data (i.e. physical units) from fixed regulatory monitoring stations and modelled forecasts into an accessible representation of air quality conditions (often, unitless values) and health effects more likely to be experienced on the day described by the AQI or soon afterwards (i.e. the short term) (37). The rationale for converting different pollutant levels (e.g. $\mathrm{NO}_2$ , $\mathrm{O}_3$ , $\mathrm{PM}_{2.5}$ ) into standardized AQI values is to reduce complexity because individual air pollutants vary in concentration ranges, measurement units (e.g. $\mu \mathrm{g} / \mathrm{m}^3$ , ppb) and concentration-response relationships. This process allows for easier comparison of the relative concentrations (and health risks) of the various pollutants using a uniform scale (e.g. ranging from 0 to 500). The mathematical formulae that are used to convert an air pollutant concentration into a unitless AQI numerical value differ around the world and are updated in accordance with changes to regulations. They generally include a linear scale (e.g. in Australia, where the AQI is calculated in direct proportion to the air quality standards), normalized or nonlinear scale (e.g. the United States AQI, where concentrations of each pollutant are converted into a 0-500 scale) and category (band) system (e.g. in the United Kingdom, where a 10-point scale is divided into low, moderate, high and very high bands) (35). Formulation of an AQI is usually based on the national air quality standards for each country. These standards are formulated based on the short-term health effects of the index pollutants, current levels of pollutants, scientific evidence emerging from toxicological and epidemiological research, developments in national limit values, and air quality objectives (37). A specific AQI value (e.g. 100 in the United States of America) is typically set at the ambient concentration for each pollutant that corresponds to the short-term National Ambient Air Quality Standard. The commonly selected air pollutants that are used to calculate AQIs include those that are currently known to have the greatest short-term health effects on the local population, namely $\mathrm{NO}_2$ , $\mathrm{O}_3$ and $\mathrm{PM}_{2.5}$ (or particulate matter of $\leq 10~\mu \mathrm{m}$ in aerodynamic diameter $(\mathrm{PM}_{10})$ ). Some countries (e.g. United States) also incorporate carbon monoxide (CO) and sulfur dioxide $(\mathrm{SO}_2)$ . Each pollutant is compared with a benchmark that represents the ideal state of air quality. At national level, individual countries determine their benchmark by accounting for factors such as level of economic development and air pollution control capacity. Most commonly, the national air quality standards of each jurisdiction are used as the benchmark for AQI construction (35). The pollutant with the highest value relative to its limit value is typically reported as the overall AQI value for the designated time. For example, if the forecast or measurement is Moderate for $\mathrm{O}_3$ and Low for $\mathrm{PM}_{10}$ , then the overall value assigned will be Moderate. Disregarding other pollutants with smaller or equivalent values means that potential cumulative or synergistic effects of multiple air pollutants cannot be captured, even though they may exist and most pollution episodes are not confined to a single pollutant (38). Averaging times to calculate AQIs often differ between pollutants (e.g. 8 hours for $\mathrm{O}_{3}$ , 24 hours for $\mathrm{PM}_{2.5}$ ), reflecting the exposure windows used in many epidemiological studies that show acute health effects (10-12). As AQI values for $\mathrm{O}_{3}$ and $\mathrm{PM}_{2.5}$ relate to relatively long averaging times, delays can occur between the onset of air pollution episodes, an AQI reaching a warning level and the issuance of behavioural advice to limit exposure. For example, the official governmental AQI for $\mathrm{PM}_{2.5}$ could not technically be reported until after the end of the day (i.e. the 24-hour averaging period is complete). To overcome this shortcoming and inform the public about air quality in approximately real time, rapidly changing AQIs are often reported (e.g. on nongovernmental internet websites). These represent AQIs calculated from pollution levels averaged (or modelled) over briefer periods, such as the last hour, or weighted towards the most recent few hours (39). Forecasted AQI values for the following hours or days are also often reported (40). An important contextual point that can be taken from such averaging times is that all AQIs in their current form are solely intended to inform and help to protect the public from relatively high levels of air pollution (i.e. concentrations above the AQI thresholds set for a country or region) that occur intermittently over the short term (i.e. hours to a day). They do not aim to inform the public about or mitigate the adverse health effects of long-term exposure to lower concentrations of air pollutants, which account for a greater public health burden and must be addressed by other means (e.g. annual AQG levels). AQIs consist of several bands or categories, sometimes named and often differing across countries, that indicate either the air quality (e.g. good or poor) or air pollution level (e.g. low or high). The level of risk to human health can be inferred because it is monotonically associated with pollution level and, thus, AQI value. The bands may be further divided into a numeric scale representing the calculated AQI value, with the format and presentation varying by country. The higher the band/number, the greater the level of air pollution, health risk and need to take precautions. These scales often incorporate colour coding to aid interpretation and use equivalent colours on maps posted on AQI websites and apps to allow users to quickly determine the local air quality (41). The setting of AQI breakpoints between bands is mainly based on (inter)national air pollution standards or guidelines and is largely an arbitrary process (35,42). Indeed, epidemiological studies have shown that, at population level, no thresholds of effect can be identified for the common air pollutants; thus, an impact can be expected for some individuals even at low levels of exposure (43-45). # 2.1.3 Health-based AQIs Limitations of single-pollutant-oriented (conventional) AQIs include having (i) the potential to poorly communicate non-threshold relationships between short-term air pollution exposures and health effects (especially when AQI categories rather than individual values are considered); (ii) an inability to capture the cumulative health risks of multiple air pollutants; and (iii) a link to regulatory standards that might be influenced by factors other than health risk. These issues have led some countries to develop health-based multipollutant indexes, or AQHIs (46-49), as follows. First, various health risks related to individual pollutants are calculated based on formulae constructed using the full range of concentration-response functions from local epidemiological analyses (e.g. time-series studies) (46). Next, the risks associated with each pollutant are combined. The specific health outcome (e.g. mortality), air pollutants and mathematical methods to combine health risks vary among AQHIs. As such, AQHIs comprehensively reflect a specific short-term health risk (e.g. daily mortality, emergency department visits for acute ischaemic stroke or asthma) for different mixtures of air pollutants. They also capture the significant gradation of health risks across low levels of exposures to multiple pollutants, even those falling below regulatory standards – which conventional AQIs fail to do (35). The AQHI values are then adjusted to a scale, further grouped into categories to reflect different levels of health risks and reported for both the current air quality situation and forecasted to provide warnings for subsequent days. In summary, AQIs provide a scale related to the concentration of the pollutant whose level most exceeds its national standard, whereas AQHIs provide a scale related to the short-term health risks of exposure to a mixture of pollutants (Fig. 1). AQ(H)Is may serve as counselling tools for the public by providing behavioural guidance linked to the index categories. Higher AQ(H)Is (i.e. worse air quality) are stepwise linked to more intense/stringent advice for behaviour changes. Typically, susceptible groups (Box 1) are advised to make behaviour changes at lower index categories (16). Examples include people with asthma, pulmonary disease and cardiovascular disease (CVD); pregnant women; older people; and children (16). Within each index category, specific health messages are tailored to the risk of adverse health effects among various populations to limit acute exposures and mitigate potential health effects. Advice during periods of poor air quality may be to reduce or reschedule outdoor activities, use public transport to reduce the number of private vehicles on the road or check that medication is being taken as advised (15). Vulnerable populations (Box 1) may also be considered during AQI development. Examples include people living in urban areas or in proximity to fossil fuel emission sources (e.g. main roads, industrial activities), people with a lower socioeconomic status and racial/ethnic minority populations (often, but not exclusively), and many citizens of low- to middle-income countries globally. Elevated AQIs can also serve as triggers for authorities to implement interventions such as traffic restrictions, free/reduced fees for public transportation, restrictions on solid fuel use for heating and limitations on high industrial emitters. # 2.1.4 Implications of the differences between AQIs and AQHIs By capturing health risks due to the combined actions of multiple air pollutants (not only the single most elevated pollutant), AQI values convey more information than AQIs to the public. This is especially relevant when some or all pollutant levels are low (below regulatory standards). AQHIs correlate more strongly than AQIs with health risks (reviewed in sections 2.2.2 and 2.3). Increases along the AQHI scale and the threshold values chosen for categories represent uniform stepwise elevations in health risks derived from epidemiological data rather than the irregular thresholds of pollution levels set by regulations for AQIs that are of no direct health relevance. Individual AQI values (e.g. 1-500) represent converted air pollutant concentrations (section 2.1.2.). Therefore, the full range of AQI values conveys indirect (and perhaps overly precise) estimations of health risks on a continuous scale. As such, adverse health risks at low exposures below thresholds derived from guidelines or regulatory standards (e.g. AQI values of 1-49 in the United States) are indirectly conveyed by conventional AQIs. However, it is highly probable (albeit not well studied) that the public pays more attention to the colour-coded AQI categories (or bands) than to exact AQI values, especially since health messages and behavioural guidance are associated with broad categories. These categories are generally formulated and named in relation to regulatory standards (e.g. "good" is used for the lowest AQI category within the United States National Ambient Air Quality Standards). A "good" AQI category could be logically misinterpreted by the public to mean that the air quality in this range poses no health risk, rather than the more accurate description of a low health risk. Conversely, by the nature of their calculation, AQHI values (as well as category names) are intrinsically risk based and, thus, explicitly communicate to the public the lack of a safe lower threshold of air pollution exposure (i.e. "low risk" is the lowest AQHI category in Canada). Despite the relative merits of AQHIs (Fig. 1), their implications for public health are not well described. A potential shortcoming is that all multipollutant AQHIs issued by governments to date (e.g. Canada) have been linked to one specific health outcome (e.g. mortality). Their ability to predict other health end-points (e.g. asthma or CVD events) may be less robust, given the differing concentration-response relationships for both a given pollutant and health end-point. Secondly, while the numerical values of AQHIs and AQIs may be discordant under similar environmental conditions (i.e. differ with respect to the percentile ranking within their own scale), the assigned categories will probably be mostly concordant (e.g. "low risk" for a Canadian AQHI versus "good" for a United States AQI) as they incorporate broad bands of values. This means that public messaging and behavioural advice may not differ between these indexes. However, it is conceivable that under some sets of conditions the combined effects of multiple pollutants (especially when levels are just below the regulatory thresholds) could elevate an AQHI but not an AQI into a higher category. It must also be remembered that the threshold values chosen for AQI and AQHI categories are subjective. Changes can and do occur, especially for AQIs when new national regulations are implemented. As such, any category discordance between indexes can change and, largely, reflects subjective threshold selections along continuous scales. Fig.1. Similarities and differences between AQIs and AQHIs # AQI - Represents a scale of air pollutant concentrations - Based on a single air pollutant that most exceeds its air quality standard - Category/band names may convey a lack of risk from low pollutant levels within regulatory standards - Inconsistent between locations with different standards or if standards are revised - Use of regulatory standards for category thresholds may imply the existence of safe levels of exposure # Both - Represent short-term pollution levels or acute health risks - Aim to inform the public about local air quality - Goals are to reduce exposures and related public health burden - Maintain public awareness of air quality as an important issue when routinely reported - No communication about long-term air pollution levels and related health effects # AQHI - Represents a scale of health risks related to specific outcomes - Based on multiple air pollutants and their combined health effects - Category/band names are risk based and communicate health risks even to low levels of exposure - Risk scale developed and adapted for use for a specific location and population (e.g. Canada) Reflects scientific evidence that no lower threshold of exposure is safe at a population level # 2.2 Existing and novel health-based AQIs # 2.2.1 Canadian AQHI as an exemplar In the early 2000s epidemiological evidence showed that adverse health impacts occur below the air quality standards used as thresholds for calculating the AQI categories in Canada (50). This brought into question the validity of the approach to characterize air quality into simple categories (e.g. good, fair, poor) as a means to improve public communication and understanding. The epidemiological evidence, which increasingly focused on $\mathsf{PM}_{2.5}$ and $\mathsf{PM}_{10}$ , could not identify threshold concentrations below which no harmful effects were observed in the population. In parallel, there was increasing recognition of the limitations of communicating air quality risk to the public via AQIs. A particular concern was that basing AQIs on the single most elevated pollutant surpassing its standard does not allow for the possibility that concurrent exposure to additional pollutants in the mixtures could further (e.g. additively) increase health risks (46,51). It is also possible that the air pollution mixture may alter the health risks posed by individual pollutants (e.g. gaseous pollutants can increase the toxicity of $\mathsf{PM}_{2.5}$ ) (52). Public health communities and the Government of Canada also noted a significant shortcoming in the communication of air pollution risk to the public. Roughly $92\%$ of premature morbidity (e.g. hospitalizations) and mortality associated with air pollution were occurring during periods when the AQI classified air quality as very good or good (53). Therefore, a new AQI approach to address some key limitations was proposed by Health Canada (Canada's Federal Health Ministry) in collaboration with Environment Canada, representatives of provincial and municipal governments, nongovernmental organizations and other stakeholders. After a few years of local pilot programmes, in 2008 Canada became the first country to officially adopt a nationwide multipollutant AQHI using short-term air pollution levels and daily mortality data from cities across the country (46,54). The modelling approach aligned with available epidemiological research showing that the combined effects of several pollutants on daily mortality consistently exceed the effects of the individual pollutants alone (55). The finalized AQHI was based on the concentrations of three pollutants: $\mathrm{NO}_2$ , $\mathrm{O}_3$ and $\mathrm{PM}_{2.5}$ (46). The inclusion of CO and $\mathrm{SO}_2$ was considered but rejected because their importance to health risk was not consistent across the country once the effects of the first three pollutants were taken into account. In practice, individual single pollutant models that quantify the excess mortality risks associated with ambient concentrations of $\mathrm{NO}_2$ , $\mathrm{O}_3$ and $\mathrm{PM}_{2.5}$ are combined additively into a single AQHI equation. The results are then transformed into a 10-point scale to facilitate public communication of the overall health risks and how they vary across the range of pollutant concentrations observed in Canada. A distinguishing feature of this novel approach is that the scale categories of 1-10 represent rising rates of predicted mortality risks (due to the combined effects of pollutants) and not simply increasing ambient pollution concentrations – with thresholds set by national standards. While all three pollutants play a role in the AQHI calculation, the underlying modelling is most driven by $\mathrm{NO}_2$ concentrations in Canada. The merits of including several pollutants (i.e. the AQHI approach) versus the conventional AQI based on the pollutant that exceeds its standard by the greatest amount has also been demonstrated for selected United States locations (56). Stakeholder input was considered throughout implementation of the AQHI. Of note was Environment Canada's (Federal Environment Ministry) successful rollout of a national ultraviolet index and forecast system (57) and its priority of enhanced environmental prediction services. With comprehensive air quality models and greater computing power, the capacity for routine hourly, multiday, multipollutant predictions was becoming possible (58). This meant that air quality, as represented by the AQHI, could be forecasted along with the traditional weather and ultraviolet radiation forecasts. These parallel objectives and the tendency for hourly air pollutant concentrations to be highly variable led to the AQHI being expressed as running 3-hour averages that could be updated throughout the day and predicted for future days. Fig. 2 is a graphic designed to inform the public on how to interpret the Canadian AQHI and its forecast values. A challenge with communicating health risk is that there is a range of susceptibility across various populations. As a result, two sets of messages are provided: one for the general population and one for people at a greater underlying health risk (e.g. patients with asthma) (59). People who believe they might be at higher personal risk are advised to obtain guidance from their doctor. This requires that the medical community is familiar with the AQHI, the adverse health effects of air pollution, and how to appropriately counsel patients at the various levels of susceptibility. # Toronto - Air Quality Health Index Fig. 2. Snapshot of the Canadian AQHI for Toronto at a given time # At-Risk Population: - Reduce or reschedule strenuous activities outdoors. Children and the elderly should also take it easy. Find out if you are at risk # General Population: - Consider reducing or rescheduling strenuous activities outdoors if you experience symptoms such as coughin and throat irritation. # Forecast Maxima Issued at: 6:00 AM EDT Thursday 31 July 2025 Thursday 7-High Risk Thursday night 4 - Moderate Risk Friday 3 - Low Risk Friday night 3 - Low Risk # Next 24 hr | Health Message # Who is at risk? People with heart and lung conditions are most affected by air pollution. To find out if you are at risk, consult the health guide, your physician, or your local health authority. Visit the national AQHI Web site to learn more about the AQHI. # Did you know...? You should limit the use of toxic paints, paint removers, stains, varnishes, waxes, glues, adhesives and cleaners on days when the air quality is high risk. # Air Quality Health Index Messages This table is a summary of air quality health messages by category <table><tr><td rowspan="2">Health Risk</td><td rowspan="2">Air Quality Health Index</td><td colspan="2">Health Messages</td></tr><tr><td>At Risk Population*</td><td>General Population</td></tr><tr><td>Low Risk</td><td>1-3</td><td>Enjoy your usual outdoor activities.</td><td>Ideal air quality for outdoor activities.</td></tr><tr><td>Moderate Risk</td><td>4-6</td><td>Consider reducing or rescheduling strenuous activities outdoors if you are experiencing symptoms.</td><td>No need to modify your usual outdoor activities unless you experience symptoms such as coughing and throat irritation.</td></tr><tr><td>High Risk</td><td>7-10</td><td>Reduce or reschedule strenuous activities outdoors. Children and the elderly should also take it easy.</td><td>Consider reducing or rescheduling strenuous activities outdoors if you experience symptoms such as coughing and throat irritation.</td></tr><tr><td>Very High Risk</td><td>Above 10</td><td>Avoid strenuous activities outdoors. Children and the elderly should also avoid outdoor physical exertion.</td><td>Reduce or reschedule strenuous activities outdoors, especially if you experience symptoms such as coughing and throat irritation.</td></tr></table> Note: the Canadian AQHI is a tool to be used by people to reduce their short-term exposure to air pollution and plan, on a daily basis, to modify their behaviour and reduce their personal health risk. (At-risk individuals/populations are determined through self-identification and/or informed by advocacy organizations or personal health practitioners.) The example shows the AQHI for Toronto at a given time. Step 1: identify the current (or forecasted) AQHI level for your location. The 1–10 scale (often stated as $1 - 10+$ because air quality subsequent to the data period used to develop the formula and scale may have conditions (i.e. $\mathrm{NO}_2$ , $\mathrm{O}_3$ and/or $\mathrm{PM}_{2.5}$ levels) that lead to AQHI values of $< 10$ ) is categorized into qualitative descriptions of risk (low, medium, high and very high) according to different ranges of AQHI values: the higher the number, the higher the health risk. Step 2: identify your health risk status (i.e. susceptibility or sensitivity to air pollution). Information for the general population and susceptible groups can be obtained from multiple online sources. With the growth of the public's use of smartphones, applications that provide weather, ultraviolet radiation and air quality forecasts have been promoted for users to customize the information based on their personal air-quality risk level. Step 3: follow the relevant health messages (59,60). Source: Environment and Climate Change Canada (61) and Government of Canada (62). Reproduced with permission from Environment and Climate Change Canada as a copy of the version available on the Air Quality Health Index and Air Quality Health Index Messages websites. # 2.2.2 Additional strengths and limitations of the Canadian AQHI The AQHI is inherently relevant for Canada because it was derived from current, locally relevant epidemiological data. While this is a strength, the underlying equation and risk categorization descriptions for the AQHI values cannot simply be adopted by other countries because the differing air pollution characteristics (e.g. pollutant levels, mixtures, sources, composition, toxicity) and population demographics (e.g. susceptibility, disease prevalence, secular event rates) can lead to different concentration-response relationships. Furthermore, how well the current AQHI (developed approximately 20 years ago) reflects variations in population risk in Canada is likely to change over time as air quality mixtures and demographics change (e.g. downward trends of some pollutants or increased prevalence of wildfire smoke). While updating AQIs as air quality standards evolve is relatively straightforward, updating the AQHI is more complex because of the need to repeat epidemiological analyses with newer data. Balancing such an update versus the need to communicate to users why the air quality appears to be different according to an updated AQHI is an ongoing consideration. A potential shortcoming of the Canadian AQHI is that it was developed based solely on the daily mortality risk. Therefore, its relationship with specific health outcomes (such as pulmonary (e.g. asthma) versus CVD events) may be different and potentially less robust, given the differences in concentration-response relationships and in the importance of individual air pollutants. While many air pollutants often track together in a mixture, it is possible that on certain days, for example, only $\mathrm{O}_3$ levels may be high. This could lead to a situation where the AQHI remains relatively low (because it is calculated based on excess mortality from three pollutants) but susceptible individuals (e.g. people with asthma) could, nonetheless, face unique health risks and are not made appropriately aware. Despite this limitation, the broader relevance of the AQHI has been somewhat validated in this regard. For example, the AQHI was found to relate to the usage of asthma health services (63), emergency room visits for stroke (64) and asthma (65), and 3-hour changes in ambulatory cardiac measures (66). Its relevance outside large urban centres has also been evaluated (67). A final limitation, which is not unique to the Canadian AQHI, is that the thresholds selected for rising categories (e.g. from low to moderate risk) are inherently subjective, given the monotonic concentration-response relationship. The simplified risk scale $(1 - 10+)$ is intended to improve public communication. However, due to how the values are determined, there is no way to reverse translate into actual ambient pollution concentrations. No interpretations of the actual health risks, in either relative (e.g. $1\%$ above ideal) or absolute terms, are conveyed. As such, there is potential for under-interpretation or over-interpretation of the health risks by individual members of the public. This represents an ongoing communication challenge. # 2.2.3 Modifying the Canadian AQHI for wildfire smoke Due to the impact of climate change and the increasing frequency of wildfires, there is growing concern about the responsiveness and applicability of the AQHI to wildfire smoke events, which are principally driven by high levels of $\mathrm{PM}_{2.5}$ (68). This led to the development of the AQHI+ formulation (69), which is implemented when $\mathrm{PM}_{2.5}$ concentrations are impacted by wildfire smoke. This tends to be when the $\mathrm{PM}_{2.5}$ concentration exceeds $30~\mu \mathrm{g} / \mathrm{m}^3$ . In such wildfire events, the AQHI+ is calculated as the $\mathrm{PM}_{2.5}$ concentration divided by 10 (irrespective of $\mathrm{NO}_2$ and $\mathrm{O}_3$ levels) and rounded up to the nearest whole number (i.e., a $\mathrm{PM}_{2.5} > 30~\mu \mathrm{g} / \mathrm{m}^3$ corresponds to an AQHI+ of 4