The Hanover Collaboration

We had a brief discussion earlier regarding the effects of exercising in cold weather on one’s pulmonary function. The basic concern is that exercise in very cold weather damages ones respiratory system, leading to impaired oxygen uptake or even “causing” exercse induced asthma (EIA).

Via personal communication, Steve Gaskill, a professor of Health and Human Performance, confirms many of my suspicions that these concerns are largely unfounded. (Disclaimer: Steve actually had this conversation with Audrey, my fiance, who relayed it to me. What follows is my summary of what he said. Any innacuracies are entirely due to me, or Audrey.)

Short answer: Cold weather exercise is not dangerous to your lungs or airways, at essentially any temperature. Cold weather exercise (contrary to popular belief) does not cause exercise induced asthma (EIA).

Long answer: First, damaging actual lung tissue with cold, dry air is essentially a physiological impossibility. If it were cold enough that you couldn’t warm the air enough before it hit your lung tissue, you’d have much bigger problems than damaged lung tissue. And in any case, even if Axel Tiechmann lost a significant percentage of his lung surface area, it would have no effect on his oxygen uptake. Our lung surface area far exceeds our ability to use it to absorb oxygen.

Exercise in cold, dry weather does irritate the tissue lining your bronchial tubes. This causes mast cells in this tissue to break or burst, releasing histamines. The histamines cause your bronchial tubes to constrict, resulting in wheezing and coughing. This is the “post-race hack” we all know. Why doesn’t this happen until after the race? Well, while you’re racing, you’re producing a lot of adrenaline (and other chemicals) that happens to bind with histamines, blocking their effect. Once the race is over, and your adrenaline levels go down, you still have all sorts of histamines floating around and bingo! Wheezing and hacking.

This physiological response I’ve described (breaking mast cells; releasing histamines; constricting bronchial tubes) is clinically indistinguishable from asthma. When we get post race hack, we are literally having a minor asthma attack.

As Tom alluded to in one of his comments, asthma and EIA are exceedingly difficult to diagnose. This is because, as I’ve noted, anyone can experience these symptoms under the right circumstances. What happens is that some people have lots of mast cells. Or their mast cells are really fragile. These people have a predisposition towards more frequent and more severe asthma attacks. This predisposition exists prior to them ever exercising outside in cold dry weather! That last part was important, so I’ll say it again:

Some people, due to natural (or inherited) variations in physiology are prone to experiencing EIA. Exercise in cold, dry air will unmask this tendency.

This means that the unusually high rates of EIA among, say, xc skiers is not unusual. It’s the activity that is unusual. If we took a large sample of people who lived only in the tropics, and forced them to exercise in cold dry air, we would see EIA at the same “elevated” rates we see in xc skiers.

But what about repeatedly irritating all that bronchial tissue and mast cells? Couldn’t that make them more likely to burst and release histamines in the future?

Not likely. Apparently, bronchial tissue is essentially the most regenerative tissue in the human body. Any cells you kill by racing in 5F weather will have grown back and be good as new within like 8-12 hours. (Brayton has pointed out that several people noticed after the Fairbanks races that the post-race hack stuck around longer than usual; specifically that going hard would immediately trigger hacking as much as a week later. I have no explanation for this.)

What does happen is that during the period where you’ve irritated your bronchial tissue, there probably is an increased risk of infection (i.e. upper respiratory colds), but after your tissue has regenerated that risk disappears. He was not entirely sure how big the increased risk of infection actually is.

It seems more plausible that race temperature cutoffs are there to protect race organizers from frostbite/hypothermia and racers from the same.