This is an excerpt from Advanced Environmental Exercise Physiology-2nd Edition by Stephen S. Cheung & Philip N. Ainslie.
It is clear, especially with acute exposure, that air pollution can pose a challenge to the respiratory and circulatory systems and thus present a major obstacle for peak athletic performance. National and world championships and large multisport competitions will remain tied to major metropolises largely because of logistics and marketing, so air pollution is likely to be an issue for many elite athletes in the coming years. During smog alert days or periods when air quality is low, extra planning is required for athletes during both training and competition (figure 13.4). In this section we outline some major considerations for planning and research by athletes and sport scientists.
Exercising in Urban Environments
Athletes training and competing outdoors may be much more susceptible to air pollutants and have a much higher effective dose than the general population or athletes primarily training and competing indoors. Besides the higher ventilation rates during exercise, the first obvious difference is in the time spent outdoors by athletes, which often also coincides with the times of peak pollution levels. In urban settings, walkers, runners, and cyclists may also be exercising near major roadways, where the dosage of pollutants may be much higher than average reported values. Whether otherwise healthy and fit persons should avoid exercise during periods of high pollution comes down to common sense; existing studies suggest there is no significant cardiorespiratory impairment at either low- or moderate-intensity exercise levels in a diesel-polluted environment. Of course, this leaves unexplored the issue of chronic exposure to high pollution levels on long-term exercise capacity, especially in children during their growth and development.
In urban environments, it seems essential that both recreational and elite athletes be educated about protecting themselves from the worst effects of pollutants while exercising. Because it is difficult to change the air that we breathe, most of the advice comes down to commonsense ways to reduce exposure. Chief among these is to exercise in quieter settings away from heavy traffic and busy roadways. If this is not possible, then exercise should be performed during nonpeak periods of traffic flow. For example, an analysis of pollutants in Toronto, Canada, demonstrated that different pollutants peak at different times throughout the day (Campbell et al. 2005). O3, particulates, and SO2 peaked at midday, while CO and NO2 peaked during the morning rush hour. Pollution levels were lowest prior to 7 a.m. and after 8 p.m.
With all the data available about the potential health threats of air pollution and its potential impact on exercise, it is only natural to pursue the question whether it is worthwhile for urban dwellers to be exercising at all. At a very basic level, this question can be posed as an issue of cost versus benefit over the course of a lifetime:
Do the potential negatives from increased exposure to air pollutants, acting primarily on the respiratory system but possibly increasing the risks of other illnesses (e.g., cardiovascular diseases, cancer), outweigh the positive health outcomes stemming from a physically active lifestyle?
One of the largest such cost–benefit analyses was conducted in Hong Kong (Wong et al. 2007) as part of a massive project on the various causes of mortality. Previous reports on the cohort had separately demonstrated that exercise decreased mortality risks and that higher air pollution levels elevated risks. Therefore, the project aimed to bridge these two reports by directly addressing the correlation between exercise levels and susceptibility to higher pollution levels. The database consisted of 24,053 individuals over 30 years old who had died in Hong Kong in 1998, representing nearly 80% of total deaths in this age group. From interviews with next of kin, exercise activity levels were obtained and broadly categorized into “never exercise” and “exercise once a month or more.” Correlations between exercise levels and daily pollution levels over 1998 showed that the group older than 65 years that never exercised had a significantly higher risk of mortality, independent of socioeconomic status, smoking history, or health status. Interestingly, the majority of exercise in the elderly group consisted of walking and tai chi. These activities are generally moderate in intensity and often take place in parks or in early morning before traffic levels become heavy. Therefore, the effective dosage of pollutants may differ substantially from models based on peak or daily levels averaged over the entire region or even localized to areas of residence.
Within the limitations of the research, interpretation of the data would suggest that moderate exercise over the life span provides protection against mortality from transient spikes in pollution levels. The mechanisms underlying such benefits are not open to investigation from such reports, but they may derive from greater respiratory clearance of pollutants with increased activity and fitness, enhanced immune function, and altered gene expression protecting against environmental damage (Wong et al. 2007).
An epidemiological approach may ultimately be best suited to teasing out the long-term health effects of outdoor pollution on athletes. As an example, one possibility may be to use sedentary and active adults within a large urban area as a cohort and from there categorize the athletic group into outdoor (e.g., runners, cyclists) and indoor (e.g., masters swimmers) subgroups to explore whether one group experiences increased risk. Comparing the health outcomes and morbidity and mortality rates between the sedentary and the combined active groups would provide information on whether exercise confers a protective health benefit despite the added pollutant exposure. Similarly, if outdoor exercise poses an additional health risk compared to indoor exercise, then we would expect runners and cyclists to experience greater morbidity and mortality rates than swimmers.
Lessons From the Olympic Games in Beijing
The hosting of the 2008 Summer Olympics in Beijing, one of the world’s most polluted megacities, instigated an effort within the sport science community to understand the potential effects of air pollution on exercise. The pollution levels in Beijing for O3, CO, SO2, NOx, and PM closely approach or exceed the standards set for long-term health in the general population by the U.S. Environmental Protection Agency, and the effects of pollution are potentially exacerbated by the high heat and humidity typical during August. The potential for safe athletic performance remained a concern for the Beijing Games Organizing Committee, the International Olympic Committee, and various nations in the years and months preceding the Games, accompanied by scientific speculation about the potential for achieving record performances. In response, the Chinese government placed extra emphasis on reducing air pollution by closing factories and limiting vehicle traffic in the months before the Games, while at the same time increasing meteorological monitoring and implementing additional medical plans for dealing with allergies and asthma in tourists and athletes.
The preparation and planning by sport agencies and athletes revolved primarily around avoiding arrival in Beijing until shortly before the competition times. Many teams traveled early to areas in or close to the same time zone as Beijing (e.g., Japan, South Korea, Singapore) for final training in the weeks before the Games. Such an approach avoided the deleterious effects of pollution but offered the opportunity to adjust the chronobiological clocks of the athletes (see chapter 14), along with providing temperatures and humidity similar to those that the athletes would encounter in Beijing. Many athletes then arrived very shortly (1 to 3 days) before their competition to avoid prolonged exposure to the pollution. This practice was not universally adopted, however. The Swiss cyclist Fabian Cancellara arrived in Beijing a full 2 weeks before the road cycling events specifically to adapt himself to the local environment, training for four or more hours daily outdoors in the Beijing area. Ultimately, Cancellara earned a bronze in the cycling road race, which lasted over 6 h, and gold in the time trial (~1 h duration). Such an accomplishment, with ventilation rates exceeding 100 L/min, demonstrates the unique ability of humans to adapt to different environmental stressors. Another option provided to many athletes by their national agencies was various masks to wear when they were not competing or training. However, the negative publicity that occurred when some American athletes were photographed wearing masks upon arrival at the Beijing airport prior to the opening ceremonies likely contributed to a quick curtailment of their use by most athletes.
The ultimate question posed by sport scientists is the effect of air pollution on sport performance. Judging from a survey of Olympic and world records, the impact on performance is arguably mixed. With events indoors, where there was some degree of air filtering along with climate control, multiple world records in both swimming and track cycling were shattered in Beijing. This would suggest that the continual improvements in training and technology were effective in improving and maximizing human capabilities. However, in outdoor running competitions, it is perhaps notable that while Olympic records were broken in many events, very few world records were broken. Olympic records were set by men in the 100, 200, 5,000, and 10,000 m events, along with the marathon and the 50 km race-walk. Of these, only the 100 and 200 m times were world records. On the women’s side, Olympic records were set in the 3,000 m steeplechase, the 10,000 m run, and the 20 km race-walk, with only the steeplechase breaking a world record. It should be noted that world record performances depend on many factors, not the least of which are race dynamics. However, given the combination of multiple world records set indoors and relatively few outdoors, it can be argued that despite optimal preparation, the environmental conditions in Beijing may have limited performance capacity—only slightly, but just enough to minimize the potential for world record performances in outdoor events.
Preparing and Planning for Pollution
Sport scientists can plan ahead by performing sophisticated modeling of the pollution patterns of a competition site, such as that which occurred before the Athens 2004 Olympics (Flouris 2006). This analysis, based on existing meteorological data for the greater Athens area from 1984 to 2003, accurately predicted that O3 and PM10 would be the most problematic pollutants during the period before and throughout the Games. The model could break down pollution levels both throughout the 24 h cycle and for different regions of the city, including the northern region where the Olympic Village was located and the downtown area where many of the venues were. Models of PM concentrations, distribution, and emission sources were also developed prior to the Beijing 2008 Olympics. Such models can be used by athletes and teams to plan the timing, location, and intensity of training sessions or to decide whether to train off-site completely and arrive shortly before competition.
To minimize pollutant exposure, athletes may consider moving some training sessions indoors, preferably to an air-conditioned facility in the summer so that temperature and humidity are controlled. Ideally, the incoming air is filtered to screen out particulates. This idea is based on the assumption that the air quality of the indoor facility is adequate and does not feature a different set of pollutants! However, research is still lacking on the relative merits of exercise indoors versus outdoors in terms of total pollutant load, during both smog alert and non-alert days. High pollution levels or the need to exercise outdoors may require the use of masks to filter out pollutants. These may range from simple gauze masks to sophisticated designs aimed at minimizing flow resistance or particular pollutants.
While it may make little sense to expose athletes unnecessarily to air pollution long-term, some short-term adaptation over 1 to 4 days before competition may help to alleviate some of the major inflammatory and respiratory responses with acute exposure to O3 and other pollutants, along with psychologically habituating the athlete to the potential discomfort posed by the pollution. Any adaptation appears to last for only a brief time, so preexposure must happen immediately before competition. It should be possible to design this environmental acclimatization into an overall tapering regimen. For example, athletes might arrive on-site 3 to 7 days prior to competition to begin their tapering. During this time, high-intensity training, with its high ventilation rates, can be performed, if possible, in a controlled environment. At the same time, passive exposure to the ambient environment during rest and recovery phases, or active exposure during lower-intensity training sessions, may be used for acclimatization purposes. However, 3 days of 2 h passive exposure to 0.20 ppm O3 did not provide any protective effect from acute exposure to higher (0.42 or 0.50 ppm) O3 levels compared to no preexposure, with the high O3 doses eliciting similar levels of spirometric impairment (e.g., FEV1) with or without preexposure (Gliner et al. 1983). Therefore, passive acclimatization to low effective doses may not ultimately translate to competition at high exercise intensities.
O3 can impair pulmonary function and potentially impair exercise capacity. It may appear that pollutants are a systemic problem whose effects can only be minimized rather than neutralized. However, besides the avoidance and management of exposure, one potential area of interest is the use of nutritional countermeasures such as vitamins and antioxidants. Because one of the proposed pathways for pollutant damage is through inflammation of cells in the lungs and respiratory passages, it has been proposed that antioxidants may minimize oxidative stress. The exact biochemical mechanism for antioxidant protection from pollution remains unclear, but it may revolve around attenuating the ozone-induced production of arachidonic acid, which may in turn also decrease central neural inhibition of ventilation.
Grievink and colleagues (1998) studied the effects of 3 months of β-carotene and vitamins C and E versus no supplementation on a group of amateur Dutch racing cyclists. Lung function measures were taken after training and after competition on 4 to 14 occasions per subject over this period, with the results regressed over the previous 8 h average ozone level, which in turn averaged 101 mg/m3 across the two groups. Pulmonary function, including FVC, FEV1, and peak expiratory flow, decreased with increasing O3 levels in the control group. This was significantly different from findings for the supplementation group, which experienced no reduction in pulmonary function with increasing O3 levels. In a subsequent study, the same research group examined a placebo versus vitamin C and E supplementation in another cohort of amateur racing cyclists (Grievink et al. 1999). Similar results were obtained, with progressive decrements in FEV1 and FVC during exposure to ozone levels of 100 mg/m3 in the placebo group but no decrements in the supplementation group. Subjects in both of these Dutch cycling studies stopped taking vitamin and mineral supplements prior to the study and abstained throughout the study; however, the typical dietary intake of the subjects was not reported, so diet could potentially have confounded the results. However, the magnitude of supplementation was relatively small at 500 to 650 mg and 75 to 100 mg for vitamins C and E, respectively, and the utility of such conclusions is that these may be less than the amounts commonly available in many supplements.
Antioxidant supplementation may also prove beneficial as short-term protection for nonathletes exposed to high levels of ozone. Mexico City street workers, tested in a placebo-crossover study, took a supplement consisting of β-carotene and vitamins C and E or a placebo (Romieu et al. 1998). Protective effects similar to those previously discussed were observed in the supplement group versus the placebo group. Interestingly, after crossing over into the second phase of the study, the group that had received the supplementation first showed lower lung function decrements than the group receiving the placebo first had shown during the first phase. This suggests that antioxidants have a washout period, resulting in a slight residual protective effect. It appears that moderate vitamin intake, either through supplementation or dietary changes, may prove valuable for athletes preparing for competition in polluted regions and also for people who are chronically exposed to elevated ozone levels.
Research Focus: Secondhand Smoke and Exercise
Clear and direct evidence shows that secondhand smoke (SHS) has long-term health impacts and increases the risk for lung cancer. Policies and legislation against indoor tobacco use in North America and parts of Europe have greatly reduced nonsmoking adults’ exposure to passive tobacco smoke. Yet when traveling for training and competitions, nonsmoking athletes can be exposed to high levels of tobacco smoke, and this could lead to acute decreases in functional capacity and performance. There is no direct research on the effects of passive smoking on exercise capacity in healthy nonsmoking adults or athletes. However, indirect evidence exists for a number of physiological changes with acute passive exposure that may relate to exercise capacity in athletes. Short-term exposure to SHS of 30 to 60 min in healthy nonsmokers can reduce coronary flow velocity (Otsuka et al. 2001) and increase thyroid hormone secretion (Metsios et al. 2007). Secondhand smoke can also reduce gonadal hormones such as testosterone, estradiol, and progesterone, and it appears to lead to elevated blood pressure and interleukin-1 levels in males but not in females (Flouris et al. 2008). Also missing from this discussion is information on the effects of chronic passive smoke exposure on exercise capacity in children and adolescents during their growth and development. Nonetheless, the copious literature on the effects of SHS makes it highly likely that the long-term potential for athletic development would be compromised in some fashion.