key: cord-0880968-xi8oovvk
authors: An, Ruopeng; Kang, Hyojung; Cao, Lianzhong; Xiang, Xiaoling
title: Engagement in outdoor physical activity under ambient fine particulate matter pollution: A risk-benefit analysis
date: 2020-10-06
journal: J Sport Health Sci
DOI: 10.1016/j.jshs.2020.09.008
sha: 63aec6f13640ea62de2ca2d0d508a009a5c0c09d
doc_id: 880968
cord_uid: xi8oovvk

OBJECTIVE: Outdoor physical activity (PA) brings important health benefits, but exposure to polluted air increases health risks. This study aimed to quantify the tradeoff of PA under fine particulate matter (PM(2.5)) air pollution by estimating the optimal PA duration under alternative pollution levels. METHODS: A risk-benefit analysis was performed to estimate the optimal outdoor moderate PA (MPA) duration under varying PM(2.5) concentrations. RESULTS: An inverse nonlinear relationship was identified between optimal MPA duration and background PM(2.5) concentration levels. When background PM(2.5) concentration increased to 186 µg/m(3), the optimal outdoor MPA duration decreased to 2.5 h/week, the minimum level recommended by current PA guidelines. When background PM(2.5) concentration further increased to 235 µg/m(3), the optimal outdoor MPA duration decreased to 1 h/week. The relationship between optimal MPA duration and background PM(2.5) concentration levels was stronger when exercising at a location closer to a source of air pollution. Compared to the general adult population, adults aged 60 years and older had a substantially steeper curve—the optimal outdoor MPA duration decreased to 2.5 h/week when background PM(2.5) concentration reached 45 µg/m(3). CONCLUSION: The health benefit of outdoor MPA by far outweighs the health risk of PM(2.5) pollution for the global average urban background concentration (22 μg/m(3)). This modeling study examined a single type of air pollutant and suffered from measurement errors and estimation uncertainties. Future research should examine other air pollutants and indoor PA, incorporate short- and mid-term health effects of MPA and air pollution into the risk-benefit analysis, and provide estimates specific for high-risk subgroups.

Physical activity (PA) is a key strategy for combatting the worldwide disease burden. [1] [2] Regular engagement in PA confers important health benefits beyond controlling risk factors for diabetes, cardiovascular disease, dementia, and certain types of cancer and can improve mood, physical functioning, and overall quality of life. 3, 4 Both the World Health Organization (WHO) and U.S. physical activity guidelines recommend at least 2.5 h of moderate-intensity physical activity (MPA) throughout the week. 5, 6 Time spent outdoors is positively associated with MPA and improves cardiorespiratory fitness. 7, 8 PA in natural settings offers non-invasive, low-cost solutions to public health problems such as mental illness and obesity. 9 Increasing time spent outdoors may be a simple and effective strategy for promoting PA and fitness in the population. 10 However, PA and outdoor air pollution may be an unhealthy combination. Currently, 91% of the world population live in places where air pollution levels exceed the limits established by the WHO air quality guidelines. 11 Fine particulate matters (PM 2.5 ), particles that are less than 2.5 micrometers in diameter, are among the main existing air pollutants. 12 PM 2.5 is a mixture of solid and liquid particles suspended in the air, most of which come from combustion of fossil fuels in the process of heating, power generation, and operating motor vehicles. 13 PM 2.5 can be inhaled and deposited in the airway and alveolar surfaces, causing local and systemic inflammation. 14 Short-and long-term exposure to PM 2.5 has been linked to elevated blood pressure, myocardial infarction and stroke, and respiratory diseases such as asthma and bronchitis. 15-17 PM 2.5 pollution is considered a significant environmental risk factor for all-cause and disease-specific mortality. [18] [19] pollution is particularly relevant to outdoor PA due to higher air pollutants inhalation during exercise. 20 Carlisle and Sharp 21 assessed the adverse impact of six major air pollutants, including PM 2.5 , on athletic performances in the United Kingdom and recommended that athletes and exercisers avoid exercising near the roadside where ambient air pollution levels are high. Some large, heavily polluted cities such as New Delhi and Beijing have taken measures to reduce air pollution and have issued warnings to discourage outdoor activities based on air quality index readings. 22, 23 On a global scale, climate change may have a profound impact on people's PA patterns. 24, 25 The interaction between global warming and air pollution may create complex dynamics that jointly influence people's PA engagement.

In the world's most polluted cities and regions, residents often need to make day-to-day decisions on whether they should exercise outdoors and for how long. The challenge of making such decisions, aided by alerts of high pollution levels, may deter people from exercising outdoors altogether, despite evidence suggesting that the benefits of exercise outweigh the health risks of air pollution in most urban environments. 26, 27 A large cohort study in Denmark found that long-term exposure to high levels of traffic-related air pollution did not attenuate the benefits of PA on premature mortality. 26 Two studies concluded that the benefits of active commuting (i.e., cycling or walking) and outdoor PA outweigh the risk of mortality due to air pollution and traffic accidents. 27, 28 More recently, Giallouros et al. 29 estimated the impacts of cycling and walking during high air pollution days on all-cause mortality. They concluded that in general, the health benefits of cycling and walking outweighed the mortality risks induced by the exposure to outdoor air pollution.

Built upon previous studies, 27, 29 our study aimed to estimate when the health benefits of PA get washed out or even reversed by the health risks of air pollution. Specifically, we conducted a risk-benefit analysis to determine the optimal outdoor MPA duration in PM 2.5 pollution. In order to be inclusive of diverse populations, we included all types of outdoor MPA rather than focusing on a specific activity, such as active commuting. We further estimated the optimal MPA duration in response to PM 2.5 pollution by PA locations. Finally, we assessed the optimal MPA duration among adults aged 60 years and older.

Risk-benefit analysis is a set of quantitative methods, drawn from different disciplines, used to comprehensively evaluate and estimate the risks and benefits of an existing or prospective action (e.g., behavior, policy, or intervention). 30 As a result, a risk-benefit ratio is often calculated to quantify the tradeoffs of risks and benefits that can be used to inform decision-making.

In this study, we conducted a risk-benefit analysis to estimate the optimal outdoor MPA duration under different PM 2.5 concentration levels (µg/m 3 ). The risk of outdoor MPA pertains to the elevated all-cause mortality risk resulting from ambient PM 2.5 pollution. In contrast, benefit pertains to the reduction in all-cause mortality risk attributable to increased PA level. We estimated the outdoor MPA duration (hours/week) that optimized the combined risks and benefits (i.e., minimized the overall mortality risk ratio) across the range (2-500 µg/m 3 ) of PM 2.5 concentration levels.

The PA exposure variable is expressed as MET-hours per week computed as the MET intensity multiplied by hours per week of a specific activity. MET refers to metabolic equivalent and is computed as the activity-specific metabolic rate divided by the resting metabolic rate. One MET is the metabolic energy expended while sitting quietly, roughly equal to 3.5 mL/kg/min. Moderate-intensity PA (3.0-5.9 METs) increases the metabolic rate 3.0-5.9 times over resting, and vigorous-intensity PA (≥6.0 METs) increases the resting metabolic rate 6 times or more over resting metabolic values. In our study, the PA exposure assessed is MPA. We assume a PA level of 3.0 METs, which is at the lower range of MPA (3.0-5.9 METs). This assumption intends to relate the PA exposure durations to casual exercises at slower paces (e.g., dog walking, walking for pleasure, or bicycling for leisure or to work). These activities could be more realistic to the average adult exerciser and fall into the daily PA recommended by PA guidelines. 6 

The risk of outdoor MPA was calculated based on the changes in the inhalation rate under ambient PM 2.5 pollution, in comparison to the alternative of not spending time in outdoor MPA. Therefore, the risk calculation considers the incremental risk of outdoor MPA but not the overall risk of PM 2.5 pollution. concentration in a neighborhood. Empirical research shows that exposure to PM 2.5 pollution during outdoor PA (e.g., running and cycling on or near city roads) is generally higher than background PM 2.5 concentration. 22 We defined the concentration factor (CF) as the ratio of exposure to PM 2.5 pollution over background PM 2.5 concentration. In Tainio et al. 27 , the mode-specific exposure concentrations were estimated by multiplying background PM 2.5 concentration by 2.0 for cycling or 1.1 for walking. In this study, we assumed CF = 1.5 in the main risk-benefit analysis and a range between 1 and 2.5 in the subgroup analysis. Therefore, exposure to PM 2.5 pollution is the product of CF and background PM 2.5 concentration (Eq.2). 

The inhaled dose with outdoor MPA (µg/week) was calculated as the sum of 3 parts-the inhale dose during sleeping, rest, and MPA (Eq.4). In particular, the inhaled dose during MPA is the product of the inhalation rate during MPA, duration of MPA per week, and exposure to PM 2.5 pollution when exercising.

We further calculate the increase in exposure to PM 2.5 pollution due to outdoor MPA relative to the baseline without any MPA in a week. Precisely, the increase in exposure to PM 2.5 pollution is defined as a proportional increase in background PM 2.5 concentration due to outdoor MPA (Eq.5).

Burnett et al. 32 estimated the impact of long-term exposure to outdoor PM 2.5 pollution on mortality at the global scale using data from 41 cohort studies conducted in 16 countries. Using data reported in Table S1 of their study, we performed a meta-analysis to estimate the pooled effect size of the all-cause mortality risk ratio in response to a 10 µg/m 3 change in background PM 2.5 concentration to be 1.089 (95% confidence interval (CI) = 1.071-1.106). We adopted this point estimate in the primary analysis and the lower and upper bound of the 95%CI in the sensitivity analysis. We calculate the change in the all-cause mortality risk ratio resulting from exposure to PM 2.5 pollution during MPA in Eq.6. 

The benefit of outdoor MPA was calculated based on the reduction in all-cause mortality risk ratio attributable to the increase in weekly MPA duration. Arem et al. 33 pooled data from multiple population-based prospective cohorts conducted in the U.S. and Europe with a total of 661,137 adults, 116,686 deaths, and a median follow-up period of 14.2 years. Cox proportional hazards regression was performed to estimate the multivariable-adjusted mortality risk ratios in response to MPA duration per week (0 weekly MPA duration as the baseline with a mortality risk ratio of 1). 33 Based on their estimates, we performed an ordinary least squares (OLS) regression in which the natural logarithmic transformed mortality risk ratio served as the dependent variable and the third-degree polynomials of weekly MPA duration served as the independent variables (Eq.7). The adjusted R-squared of the estimated OLS was above 0.99. Fig.   1 shows the estimated regression curve. We used the estimated Eq.7 to calculate the change in the all-cause mortality risk ratio in response to variations in weekly MPA duration. The specific values for the intercept and coefficients are reported in Table 1 .

The overall all-cause mortality risk ratio, defined as the product of the mortality risk ratio in response to MPA duration calculated from Eq.6 and the mortality risk ratio in response to PM 2.5 pollution calculated from Eq.7, is shown in Eq.8. We estimated the optimal weekly MPA duration that minimized the overall mortality risk ratio at each background PM 2.5 concentration level (from 2 µg/m 3 to 500 µg/m 3 in an increment of 1 µg/m 3 in each iteration). Following the method used by Burnett et al. 32 , we adopted 2 µg/m 3 as the start point for the background PM 2.5 concentration level because that was the minimum PM 2.5 concentration level observed in the cohort studies. The equations were solved using the built-in function "optimize" in R version 4.0.

We used the lower (i.e., 1.071) and upper (i.e., 1.106) bound of the 95%CI for the all-cause mortality risk ratio in response to a 10 µg/m 3 change in background PM 2.5 concentration to estimate the optimal weekly MPA duration. 32 This sensitivity analysis offered insights into the level of uncertainty associated with the findings from the primary analysis.

Besides the primary risk-benefit analysis, we performed two subgroup analyses. First, we varied the value of CF from 1 to 2.5 (e.g., outdoor MPA in the backyard vs. near or on busy roads). 27 This analysis provided a range of estimates specific to exercise locations where PM 2.5 concentration deviates from the background average. Second, given the increasing vulnerability of older adults to elevated air pollution levels, we estimated the optimal weekly MPA duration specific to the older population. Burnett et al. 32 reported the mortality risk ratio in response to a 10 µg/m 3 change in background PM 2.5 concentration to be 1.440 among adults aged 60 years and older. This estimate was used for the analysis that was specific to older adults. Parameter values used in the primary and subgroup analysis are summarized in Table 1 . 

Engaging in PA in polluted air creates a dilemma since one should weigh the health benefits of PA against the health risks of air pollution. A growing body of literature suggests that air pollution discourages people from engaging in outdoor activities through impaired exercise capacity and performance due to decreased lung function, elevated blood pressure, [34] [35] [36] and other cardiovascular and respiratory symptoms while exercising in polluted air. 15, 16, 37 Appearance of smog and other visible pollutants can also deter people from being outdoors. 38 In addition, alerts and warnings of poor air quality from the news and public affairs programs have increased public awareness of air pollution. 39 The effects of PM 2.5 pollution on optimal outdoor MPA duration differed by exercise location. MPA engagement close to a major source of PM 2.5 pollution significantly reduced the optimal outdoor MPA duration through increased exposure to air pollution. Andersen et al. 26 (2015) found that cyclists were, on average, exposed to twice the amount of air pollution as the background (i.e., area-average) concentration level. Given these findings, people should try to avoid heavily polluted areas such as busy roads, construction fields, and air pollutant-emitting factories when exercising. 45 On the other hand, policymakers should make efforts to reduce air pollution levels in urban areas where people often exercise.

The relationship between the optimal duration of outdoor MPA and PM 2.5 concentration tends to be substantially stronger among older adults aged 60 years and older. Evidence from existing studies has demonstrated that older adults are more susceptible to air pollution-induced health effects compared to younger adults or the general population. 46 In particular, older adults are documented to be more vulnerable to particulate matter than to other types of air pollutants, with specific effects sometimes resulting in acute hospitalization and cardio-respiratory mortality. 47 The ongoing COVID-19 pandemic may have profoundly impacted people's PA. 48 A study of adult Fitbit® users worldwide found that daily PA was reduced by 10%-20% across countries. 49 In the meantime, air pollution levels have declined in many parts of the world during the pandemic. 49 The dynamic relationship among COVID-19, air pollution, and PA warrants further investigation because such studies may hold the key to knowing when to intervene and help people safely engage in adequate PA under the pandemic. 50 A few policy recommendations may emerge from our findings. First, the message that government agencies convey to the general public-that aiming for or maintaining a weekly PA level following the WHO's PA guidelines (e.g., at least 2.5 hours of MPA throughout the week)-should be safe. Following the guidelines represents a desirable lifestyle choice because the health benefit of PA is larger than the health risk of air pollution on most days. Second, high-volume exercisers and professional athletes whose weekly exercise level is considerably higher than a few folds (e.g., 3-5 folds) above the recommended minimum level given by PA guidelines should pay close attention to elevated air pollution if they wish to minimize its adverse health effect. Some safeguarding measures may be taken, such as wearing masks to filter the particulates, avoiding highly polluted roadways, or switching to indoor facilities with air purifiers. Third, older adults aged 60 years and older are substantially more susceptible to the impact of PM 2.5 pollution than the general adult population. For this population, outdoor MPA may need to be reduced or switched to indoor PA in order to avoid severe health consequences if ambient air pollution levels increase to an alarmingly high level (e.g., more than threefold the global urban average PM 2.5 concentration level).

Our study has several limitations. First, people's PA levels are primarily constrained by the resources (e.g., availability of parks and exercise facilities) and leisure time they have. 51, 52 Our study assumed that people could freely choose their location and duration of outdoor PA, but in reality most people have little choice when and where they do their daily PA. Second, our study only considered one particular type of air pollutant, namely PM 2.5 , given its high prevalence in urban settings and detrimental health impact. In contrast, other air pollutants, such as nitrogen dioxide (NO 2 ), sulfur dioxide (SO 2 ), and PM 10 , were not modeled, nor were overall air pollution levels (usually measured by some air-quality indices). In some neighborhoods, the overall air pollution level might be high despite a relatively low PM 2.5 concentration; in these cases, the concentration levels. We were unable to model those subpopulations due to the lack of data on their specific mortality risk ratios in response to MPA duration and air pollution concentration.

Given the multitude of sources of uncertainty associated with the calculation of the optimal MPA duration, it would be an adequate choice for the majority of young and middle-aged people to follow the WHO's PA guidelines. Moreover, even with a reasonably high PM 2.5 concentration level (e.g., ~180 µg/m 3 ), it would still be safe for the majority of people, except for those aged 60 years and older or with existing chronic conditions, to engage in MPA outside for at least 2.5 h per week, although outdoor PA locations could make a difference in the optimal MPA duration.

Our model assumed the same rate of reduction in mortality risk associated with MPA for the general adult population and older adults, as we did not identify concurrent large-scale meta-analyses at the global scale on age-group-specific estimates. Woodcock et al. 53 

This study estimated the optimal duration of outdoor MPA under varying PM 2.5 concentration levels and explored whether the optimal duration changed by exercise locations and age.

Although the optimal duration of outdoor MPA was inversely associated with background PM 2.5 concentration levels, the optimal duration still met the guidelines-recommended MPA of 2.5

hours per week at a concentration level as high as 186 µg/m 3 . Optimal duration decreased when PA took place closer to a major source of air pollution. Adults aged 60 years and older were substantially more susceptible to the impact of PM 2.5 pollution than the general adult population and therefore may need to reduce their duration of MPA under elevated PM 2.5 exposure. Future research should examine other air pollutants and indoor PA, incorporate short-and mid-term health effects of MPA and air pollution into the risk-benefit analysis, and provide estimates specific to high-risk population subgroups.

RA designed the study, compiled the data, constructed the simulation model, and wrote the manuscript; HK contributed to the simulation model construction and optimization; LC and XX contributed to interpreting the modeling results and revising the manuscript. All authors have read and approved the final version of the manuscript, and agree with the order of presentation of the authors. 

Outdoor physical activity (PA) brings important health benefits but exposure to polluted air increases health risks. This study aimed to quantify the tradeoff of PA under fine particulate matter (PM 2.5 ) air pollution by estimating the optimal PA duration under alternative pollution levels. A risk-benefit analysis was performed to estimate the optimal outdoor moderate PA (MPA) duration under varying PM 2.5 concentrations. An inverse nonlinear relationship was identified between optimal MPA duration and background PM 2.5 concentration levels. When background PM 2.5 concentration increased to 186 µg/m 3 , the optimal outdoor MPA duration decreased to 2.5 hours/week, the current PA guidelines-recommended minimum level. When background PM 2.5 concentration further increased to 235 µg/m 3 , the optimal outdoor MPA duration decreased to 1 hour/week.

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The authors declare that they have no competing interests.