Exercise and Pollution

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From Ace certified news Volume 10 Number 4. June/July 2004
Reprinted with permission from the American Council on Exercise (www.acefitness.org)

Many athletes and exercise enthusiasts live in densely populated urban areas where they are increasingly confronted with challenges related to air pollution caused by traffic and industry. During times of temperature inversion (when temperature increases with altitude, in contrast to the more typical decrease) or when air movement is low, air pollutants can reach concentrations that can severely impede physical performance.

The most common air pollutants are carbon monoxide, sulfur oxides, nitrogen oxides, ozone, peroxyacetyl nitrate, aerosols, soot, dust and smoke. The effect of these pollutants is, in part, related to their penetration into the body. The presence of more than one pollutant, or other environmental stressors (e.g. heat, cold and altitude), which is generally the case in most smog conditions, usually has a more powerful effect on the body. As they are inhaled, the main effects of air pollutants are on the respiratory tract. The nose hairs remove large particles and highly soluble gases very effectively (e.g., 99.9 percent of inhaled sulfur dioxide is removed in the nose), but smaller particles and agents with low solubility pass easily. During exercise, when mouth breathing plays an important role, this air filtration process is much less efficient, and more pollutants reach the lungs.

With respect to the short-term effects of pollutants on exercise performance, the main problems are irritation of the upper respiratory tract, respiratory discomfort and reductions in the oxygen transport capacity of the blood. Carbon monoxide (CO) emissions in urban areas are greater than emissions of all other pollutants combined. CO primarily affects exercise performance through its strong (200 times stronger than that of oxygen) capacity to bind to hemoglobin (COHb) in the blood, thereby reducing the blood’s capacity to transport oxygen to the tissues. Very high levels of COHb are needed to produce reductions in submaximal exercise performance. Therefore, under realistic outdoor conditions, the effects of CO only become evident when maximal exercise performance is an issue. For example, maximal oxygen uptake is reduced at COHb concentrations above 4.3 percent (during prolonged exposure to heavy traffic, COHb concentrations of 5 percent have been observed).

For those with cardiovascular impairment, problems may occur during submaximal exercise at lower concentrations of COHb (2.5 percent to 3 percent). Sulfur oxides (Sox), mainly in the form of sulfur dioxide (SO2), exert their influence through irritation of the upper respiratory tract, which can cause reflexive bronchoconstriction and increased airway resistance. Nose breathing strongly reduces this effect compared with mouth breathing. For submaximal exercise, the threshold level before pulmonary function is compromised is between one and three parts per million (ppm). For maximal exercise, no cutoffs are as yet available. For asthmatics, the threshold values for eliciting a bronchoconstrictor response are lower (0.2 to 0.5 ppm of SO2). Of the nitrogen oxides (NOx), only the effect of nitrogen dioxide (NO2) has been studied in humans. Acute exposure to high concentrations of NO2 (200 to 4,000 ppm) is extremely dangerous and has resulted in several reported deaths. During submaximal exercise, no effect of NO2 levels up to 1 to 2 ppm has been observed, but effects of higher concentrations and/or its effects during maximal exercise have not been studied.

Another pollutant that may create a health risk is ozone (O3). During light-to-moderate submaximal exercise lasting several hours, exposures to 0.3 to 0.45 ppm O3 have resulted in decrements in pulmonary function and increased subjective discomfort. For more intense levels of exercise, the respiratory discomfort can become severe and thereby limit performance. Ozone has also been associated with eye irritation, general respiratory discomfort and nausea.

Effects of aerosols on physiological function are usually caused by their effect as airway irritants. The most common aerosols are sulfates (minimal adverse effects), sulfuric acids (minimal effect, unless prolonged exposure, larger particles, and/or high ambient humidity are present), nitrate aerosols (minimal effect), and saturated and unsaturated aldehydes (e.g., formaldehyde, acrolein and crotonaldehyde), which are also irritants having minimal effect. The effects of minute particles of soot, dust and smoke on exercising humans have not been evaluated.

Generally, particulate inhalation results in bronchoconstriction. The penetration of these particulates in the respiratory system is related to the size of the particle. Below 3 microns, they can reach alveoli, between 3 and 5 microns they usually settle in the upper respiratory tract and above 5 microns they are not able to enter the respiratory tract. Thus, particles smaller than 5 microns can cause pulmonary inflammation, congestion or ulceration. Exercise, by increasing respiratory rate, may aggravate the contamination of the lungs. When exercise is to be performed in a high pollution area, valuable information may be acquired from local meteorologists. To minimize potential problems, activities should be carefully planned, taking into consideration daily and seasonal fluctuations in pollution:

• Avoid exercising during rush hours (the CO level peaks during rush hours).

• Avoid high cigarette smoking areas prior to and during exercise.

• Avoid combinations of high temperature, humidity and air pollution.

• Keep the amount of time spent in high pollution areas to a minimum (physiological effects of air pollution are both time and dose dependent).

• Be aware of seasonal variations in ozone levels. The ozone level is usually low in winter, increases during summer with a daily peak around 3 p.m., and reaches maximal values in early autumn.

Cedric X. Bryant, Ph.D., ACE Chief Exercise Physiologist



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date: October 3, 2004


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