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The American Hearth Association acknowledged this in 2004.

While exercising, you take in up to 20 times more air than sedentary person. This effectively increases body exposure to any toxin, or pollutant present in the air by as much. In other words, what is normally considered to be safe level of pollution,

can easily turn into dangerous and harmful while exercising.

For instance, the maximum safe PM2.5 (particles smaller than 2.5 microns) standard, as 24-hour average, is set by the Environmental Protection Agency (EPA) at 15μg/m3. At 15-fold the sedentary air intake, it effectively becomes 225μg/m3, considered by the EPA to be "unhealthy" 1-hour average, and quite close to the "very unhealthy" level, beginning at 250μg/m3. 

How this dramatic effective increase in air pollution due to exercising may affect your health depends on several factors, including your air quality level, frequency, duration and intensity of exercise, as well as your individual vulnerability. While short-term effects shouldn't be worrisome for most folks, long-term effect is a big unknown. Even the EPA's safe level is not tested long-term, and may not be harmless for all.

It may parallel, say, vitamin C DRI (~100mg/day), which is pretty much guaranteed to safeguard you from scurvy, but puts your health at a serious risk from compromising your health, being grossly bellow the longer-term optimum, estimated at anywhere from 400mg to 5 grams a day.

And those exercising indoors are not out of the woods either. In fact, indoor air is on average 5-10 times more contaminated than outdoor air. Air-purifiers are efficient for larger particles, but most become increasingly inefficient as the particle size goes bellow 0.1μ (they can be smaller than 0.01μ). And it is

exactly those smallest particles -

so called "ultra fine" and "nano" particles - that can penetrate through the lungs into the blood, with the potential of causing, or contributing to vasoactive (constriction/dilation) and atherosclerotic blood vessel changes.

Considering high average level of indoor air contamination,

indoor exercising is probably more unhealthy

than outdoor's, despite the use of air-purifying systems. Special concern are commonly high levels of the volatile organic compounds.

In conclusion, we simply cannot ignore air quality, even for our regular daily activities, and especially when it comes to regular exercise. Exercising in heavily polluted air is highly undesirable, and can't be considered healthful. The problem for many is that the air is polluted in their wider area. In such case, it is recommendable to adjust (scale down) exercise frequency and intensity according to the pollution level. R