Air Quality and Outdoor Cycling & Training

By Sarah Kaufmann — What are the health risks of training in bad air? My agenda with looking into this research is in evaluating the health risks for a well-trained population including myself and my community. Before I get into the details, I need to state a few caveats up front. I’m not a researcher. I’m not a scientist. I’m not a doctor. I’m a cycling coach. I don’t have the academic tools or background to suggest that these paragraphs are anything more than my review of the literature.

All the research on the well-trained (usually runners) has been focused on performance impacts, rather than health. This is valuable information; however, I’m less interested in the performance implications and more interested in the health implications, which I will review here.

Background

I think most people reading this are familiar with our local challenges with air quality here along Utah’s Wasatch Front and throughout the West where we have basins and bad winter air. Due to a combination of industrial activities, vehicle emissions, and geographic factors, pollutants become trapped, especially during winter inversions.

Air Quality Index

The Air Quality Index (AQI) is a standardized system used to measure and report air quality (see https://www.airnow.gov/aqi/aqi-basics/). It is measured in micrograms per cubic meter (µg/m3) of various types of pollutants, like particulate matter (PM – PM2.5 and PM10 are the most common), ozone, carbon monoxide, sulfur dioxide, and nitrogen dioxide. It standardizes these pollutants into a single number on a scale from 0 to 500 (See Table from Airnow). One can see local real time visual representations of the AQI on Purple Air, IQ Air, and others.

The Research

I looked through 22 relevant studies. In this review, I included information from 14 of them (see references below). Of course, it’s hard to control specific numbers or ranges around air quality exposure. ‘Exercise’ means different things in different populations and settings. So, there were some studies that I flagged immediately as the population or methods were questionable or not applicable.

I looked specifically for studies examining exposure to poor air during physical activity. A lot of these studies compared sedentary folks to people who exercised but the amount of exercise was either unclear or it was clearly not applicable for a well-trained population. I did not find any studies examining the health impacts for a well-trained population.

Some studies found that moderate exercise counteracted some of the negative consequences of exposure to pollutants. Many studies simply concluded, more research is needed, or offered vague recommendations along the lines of, ‘reducing pollution will limit health consequences.’

There is still much to learn

Many studies and meta-analyses did show that exposure to pollutants resulted in compromised health outcomes. They covered pollutants ranging from ozone, diesel exhaust, traffic-related pollution, and PM. The health consequences associated with these pollutants include an increased risk of airway inflammation, changes in blood pressure, decrements in pulmonary function, artery function, micro-vascular function, cardiopulmonary function, and immune function. Exercise performance was also shown to decline, which could lead to further, related health consequences (1, 6, 8, 12).

However, the nature of these studies and meta-analyses precludes a controlled setting. These are different populations, different levels of activity, and different and varying levels and types of pollutants. There is movement in the area of measuring exposure. For example, one study cited that there may be opportunities in the future to estimate the inspired dose (or the amount of a specific pollutant or mixture of pollutants inhaled over a given time period) which is likely a better measure than area-specific AQI (13).

The question for a general population of primarily sedentary individuals is, is some light level of activity/exercise better for a sedentary person if it comes alongside exposure to pollutants? And some of the research suggests that for that population, exercise in some level of air pollution is better than inactivity (9, 10). However, for a well-trained, very active population, the balance shifts and the exposure to pollution becomes more dramatic (inspired-dose dependent) and the downside of not exercising is less unfavorable (4). That’s the hole in which the well-trained population resides and specific research in this area, as it pertains to health outcomes, was limited.

Longer Term

There was a seemingly endless list of studies and meta-analyses showing the connection between PM2.5 and PM10 and lung cancer. Particulates enter the respiratory system through the airways and accumulate in the lungs (5). Less clear cut, some studies also showed connections between these particulates and brain impairments. Particulates enter through the airways, access the respiratory system, cross the alveolar-capillary barrier into the circulatory system, where they can penetrate the blood-brain barrier. And/or particulates enter through the nasal cavity and access the central nervous system directly through the olfactory nerve pathways. One study showed a specific link between PM2.5 and dementia (14).

Does Masking Help?

I didn’t find anything that specifically examined the effect of masking on limiting inspired dose during exercise. I did find research showing the impact on masking on exercise performance (not related to how masking impacted pollutant exposure) (7, 12). I also found research showing reductions in hospitalizations related to smoke and particulate exposure in a general population (not related to exercise) when masking was employed (7).

Unsurprisingly N95 masks showed the biggest impact in limiting hospitalizations due to smoke, on the order of ~30%, as compared to surgical or synthetic masks, which accounted for ~15% and ~12% decreases respectively. I did not find research that compared the impact of masking on performance with regard to different types of masks. I personally haven’t tried training in an N95, but I can imagine it would come with more challenges than surgical or synthetic masks since it is less porous.

Based on this, it stands to reason that masking during exercise would limit pollutant inspired dose. And based on the research around masking and performance, we accept that there will be an expected compromise on exercise performance, both in physiological and psychological symptoms (like gas exchange, pulmonary function, and subjective discomfort) (2).

It came down to this

Of all the studies I looked through, there was one that got the closest to what I was looking for and didn’t have any obvious issues with methods or parameters (3). This one got the most granular with the info and recommendations and was the most specific to our population, exposure, and uses.

This study was conducted on adults mostly aged 50-70 in China who walked or performed more vigorous physical activity (though they do not define ‘vigorous’). They were looking to find the threshold at which the benefits of exercise were offset by the exposure to PM2.5 (as measured by peak expiratory flow or the max speed at which a person can expel air from their lungs). They concluded that, on average, the detrimental effects of PM2.5 on peak expiratory flow was on the order of >77 for ‘vigorous’ physical activity and >81 for walking. So we have a threshold recommendation for training versus not training (3). Again, it’s unclear how vigorous physical activity is defined in this study. However, it’s safe to assume that when most of us go out to train, it could be considered vigorous.

Now it is a bit of a stretch to let peak expiratory flow be a stand-in for negative health consequences. That is essentially what I am doing here. However, I set out to find a number, a threshold around which to base decisions around outdoor training. And using the observed changes in peak expiratory flow, this is as close as I got to a clearcut recommendation in that area.

What does this all mean in practice?

Over the last handful of years, I have usually used an AQI threshold of 100 (the unhealthy level for sensitive groups) as my personal cutoff for outdoor training. But I didn’t have a reason for that number, it just seemed appropriate – it’s a nice round number and all, and it’s where I start to notice some acute symptoms (like a scratchy throat, watery eyes, and headaches). And that was why I set out to look for an AQI threshold based on research. My search also made it clear that more research is needed on the effects of air pollution on those training outdoors, and in particular the well-trained athlete. With all of this in mind and based on the changes in peak expiratory flow (see above and reference 3), I will likely lower my AQI threshold to 80.

How you approach training outdoors in poor air quality is wholly up to you. But hopefully you now have more information to make an informed choice.

Sources:

1. Cusick M, Rowland ST, DeFelice N. Impact of air pollution on running performance. Sci Rep. 2023 Feb 1;13(1):1832. doi: 10.1038/s41598-023-28802-x. PMID: 36725956; PMCID: PMC9892497.
2. Das A, Azarudheen S, Chandrasekaran B, Fernandes S, Davis F. The plausible effects of wearing face masks on sports performance – A scoping review. Sci Sports. 2023 Jun 28. doi: 10.1016/j.scispo.2022.12.006. Epub ahead of print. PMID: 38620146; PMCID: PMC10300654.
3. Guo Q, Zhao Y, Zhao J, Qian L, Bian M, Xue T, Zhang JJ, Duan X. Identifying the threshold of outdoor PM2.5 reversing the beneficial association between physical activity and lung function: A national longitudinal study in China. Sci Total Environ. 2022 Sep 15;839:156138. doi: 10.1016/j.scitotenv.2022.156138. Epub 2022 May 21. PMID: 35605854.
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