title: Should people be physically active outdoors on smog alert days?
abstract: BACKGROUND
  Given the importance of physical activity to well-being, there is a need to encourage people to be physically active year-round. At the same time, many people are vulnerable to adverse health effects from air pollution, especially on smog alert days. This study was undertaken to determine when air pollution levels tend to be lowest so that the public can modify strenuous outdoor activity accordingly.
  
  
  METHODS
  Existing hourly air pollution data for Toronto were analyzed to determine how pollutant levels varied from hour to hour throughout each 24-hour day, to identify the times when pollution levels are at their lowest on average.
  
  
  RESULTS
  Pollutant levels vary throughout the day, with concentrations of some pollutants (such as ozone, particles and sulphur dioxide) being highest during mid-day, and others (such as carbon monoxide and nitrogen dioxide) being highest with morning rush hour. Overall, pollutant concentrations tend to be lowest before seven a.m. and after eight p.m.
  
  
  CONCLUSIONS
  The public should be encouraged to maintain regular physical activity outdoors while monitoring any air pollution-related symptoms. The intensity of outdoor activity should be reduced, or activities replaced with indoor exercise, at those Air Quality Index (AQI) levels that trigger individual symptoms and when AQI values exceed 50. Where possible, strenuous activity should be taken when and where air pollution levels tend to be lowest, namely early in the morning and in low-traffic areas. More research is required to guide development of health protective advice on exercising when air quality is poor.
authors.affiliation: Toronto Public Health, 277 Victoria Street, 7th Floor, Toronto, ON M5B 1W2. mcampbe2@toronto.ca
authors.forename: Monica E
authors.initials: ME
authors.lastname: Campbell
authors.forename: Qian
authors.initials: Q
authors.lastname: Li
authors.forename: Sarah E
authors.initials: SE
authors.lastname: Gingrich
authors.forename: Ronald G
authors.initials: RG
authors.lastname: Macfarlane
authors.forename: Shouquan
authors.initials: S
authors.lastname: Cheng
mesh_terms: D004781:Environmental Exposure / Q000517:prevention & control*; D015444:Exercise; D017408:Guidelines as Topic; D006293:Health Promotion; D006801:Humans; D009864:Ontario; D010507:Periodicity; D012621:Seasons; D012905:Smog
chemical_list: D012905:Smog