“It was the best of times, it was the worst of times (to exercise!)”
Joe Osborne, Legal Director
We all know that getting regular exercise is important to stay healthy, and that breathing polluted air is unhealthy. As our oxygen requirements increase during strenuous exercise, our breathing rate increases to meet this increased oxygen demand. As a result, our exposure to air pollution increases as well.
You may be thinking that this is exactly the excuse you’ve been looking for to adopt an entirely sedentary lifestyle. While the problem of increased air pollution exposure during physical activity is real, replacing these health risks with the health risks associated with prolonged physical inactivity isn’t a solution.
Instead, individuals should seek out convenient ways to minimize their exposure to air pollution during exercise. That doesn’t mean wear a gas mask, but before you go jogging, you might want to check the current air quality in your area by visiting www.airnow.gov.
If air quality is poor, consider exercising indoors or waiting a few hours to see if air quality improves. As I’ll discuss in more detail in a future article, you can also significantly reduce your exposure to traffic-related air pollution by selecting jogging or cycling routes that avoid major roads when possible.
Air pollution concentrations tend to follow relatively predictable temporal patterns. Perhaps the simplest way to minimize exposure to air pollution during exercise is to familiarize yourself with these patterns. Once you know when air pollution is most and least likely to be a problem, you can adjust your routine to avoid outdoor during the times when air quality is poor.
Many parts of Pennsylvania (along with much of the eastern United States) struggle to meet federal health-based standards for two pollutants: particulate matter and ozone. By looking at air quality monitor data for these pollutants, it’s easy to spot some temporal trends in pollutant concentrations. Below I’ll use hourly average pollution concentrations measured at the air quality monitor site in Pittsburgh’s Lawrenceville neighborhood to point some of these patterns out. While this monitor data is from Pittsburgh, these air pollution trends are not unique to the Steel City. These same air pollution trends and the resulting advice about when to exercise hold true just about everywhere.
The chart below lists the number of times in 2008 the hourly average ozone concentration at the Lawrenceville air quality monitor reached or exceeded .075 parts per million (ppm).
Hourly ozone concentrations at or above .075 ppm only occurred between April and September. This is because ozone is formed from the reaction of other pollutants (most notably nitrogen oxides and volatile organic compounds) in the presence of sunlight. Less sun means less ozone—one bright side to those short, gloomy winter days.
Rather than focusing only on concentrations of .075 ppm or greater, you can see the same summertime increase by looking at average hourly ozone concentrations by month:
Now let’s look at ozone concentrations by hour during the April to September ozone season:
As you might expect given the relationship between sunlight and ozone formation, ozone concentrations are highest during and shortly after the sunniest part of the day. So the lessons for ozone are clear. From late fall to early spring, ozone is rarely, if ever a concern. During ozone season, ozone levels are usually at their highest in mid- to late-afternoon.
Like ozone, particulate matter (PM2.5) concentrations also vary by season, though not as drastically as ozone. Particulate matter can be a problem any time during the year, but unhealthy PM2.5 concentrations are most common between June and September.
The following charts are based on 2006-2008 PM2.5 data from Pittsburgh’s Lawrenceville air quality monitor.
The chart below shows the percentage of hours by month where 1-hour average PM2.5 concentrations were at or above the federal short-term standard.
Like ozone, PM2.5 concentrations are most likely to be elevated between late spring and early fall.
So what times of day are PM2.5 concentrations most likely to be elevated? The following chart depicts average hourly PM2.5 concentrations from June to September.
Early-morning (~6-8 am) and late-evening (~8-9 pm) bumps are evident. These hourly patterns can be largely attributed to two causes: increased mobile source emissions during rush hour and temperature inversions. Normally, air temperature decreases as altitude increases. During an inversion, this pattern is reversed, cutting off convection and trapping local air pollution rather than allowing it to rise and disperse. If you ever step outside during the late night or early morning and notice a pervasive sulfur odor in the air, chances are an inversion is to blame.
Elevated PM2.5 concentrations can occur any time of the year, but, like ozone, are most common from the late spring to early fall. Unlike ozone, PM2.5 concentrations are most likely to be high late at night and in the early morning.
Putting it all together
While unhealthy air quality can occur at any time of the year, those seeking to reduce their exposure to air pollution during outdoor physical activities should be most mindful of current air quality from late spring through early fall
When it comes to identifying the best time of day to exercise, ozone and PM2.5 concentrations don’t match up so conveniently. The chart below shows the percentage of hourly averages for PM2.5 and ozone in excess of the short-term standard for each pollutant:
Given the divergent hourly patterns of PM2.5 and ozone, there are only a few portions of the day when concentrations of both pollutants are unlikely to be elevated. Best bets are to try to plan outdoor exercise between 9 am and 1 pm or 6 pm and 8 pm. Keep in mind, these are just general trends. Air quality can be good (or bad) at any time of the day or year. It’s always best to check the current air quality before exercising outdoors.
Again, I’m not suggesting you reduce your general level of physical activity. It’s far from clear under what circumstances the numerous positive health effects of exercise are outweighed by the negative health effects of increased exposure to air pollution, but if you reduce your exposure to air pollution by reducing your level of physical activity, there’s a good chance you’re doing yourself more harm than good. Use this information to increase the amount of time you spend exercising when air quality is good, not to decrease the amount of time you spend exercising in general.
Back to post  Among the few exceptions to this general pattern are areas where air pollution concentrations are dominated by an intermittent local pollution source. Seasonal variations are, of course, based on seasons, so annual trends would be offset by ~6 months in the southern hemisphere and muted or nonexistent near the equator.
Back to post  Why .075 ppm? Because the most recent short-term national ambient air quality standard (NAAQS) for ozone is set at .075 ppm. averaged over 8 hours. I use the hourly averages for ozone because an 8-hour average would mask the temporal variation in ozone concentrations, not to second-guess EPA’s decision that 8-hour averages are most relevant to human health effects.
Does this mean you’d have to jog for 8-hours straight in air exceeding the NAAQS standard? No, the same document in which EPA justifies the 8-hour standard also acknowledges that exercise can increase the sensitivity to and adverse health effects of ozone, meaning multi-hour exposures to high ozone levels are more likely to result in adverse health effects if an individual is physically active during a portion of that multi-hour exposure). U.S. EPA, Air Quality Criteria for Ozone and Related Photochemical Oxidants (Vol. I) page E-22, available at: http://oaspub.epa.gov/eims/eimscomm.getfile?p_download_id=456384.
Back to post  The NAAQS standard is 35 microgram per cubic meter (μg/m3) averaged over 24-hours. As I stated above regarding the 8-hour averaging time for the ozone standard, I’m focusing on hourly averages for PM2.5 because it provides a better sense of temporal trends, not to second guess the federal PM2.5 standard’s 24-hour averaging time.