Running in smoggy conditions: How dangerous is it?


Each summer I get confused because some people say when the smog is bad I shouldn't run, while others say as long as I run in the morning the pollution hasn't built up yet. What should I do? Is concern over air pollution a scam or a real danger?


There is good reason to be concerned about smog. Polluted air contains numerous toxic compounds that have a negative effect on exercise performance.

They can cause symptoms that begin when your throat tickles, then becomes rough and scratchy. You may have chest pain, begin coughing, become short of breath, and develop a headache.

After a few days' exposure most of the symptoms decrease or go away, which makes some people think they can now handle the bad air and it isn't a problem.

This is a gamble, because hidden damage continues. We know this because of a test called a methacholine challenge. This drug narrows lung airways, and the effect is increased when the air contains ozone (the most toxic component of smog).

This increase in airway narrowing does not decrease during days of repeated exposure to ozone, which means this form of lung damage continues.

Smog is usually a problem only a few days a year. Does the damage repair before the following summer, or does it build up and gradually increase over the years?

We don't know the answers to these questions, but there's some evidence from animal studies that ozone damage doesn't heal completely.

There are several sources of air pollution, but by far the most important factor in smog formation is car and truck exhausts. Chemical reactions are triggered by sunlight and end up producing ozone, to go along with carbon monoxide and other harmful gases.

As the sun rises and sunlight grows stronger, the concentration of ozone increases, so it grows during the day. After sunset no more ozone can form, so the concentration goes down as this very active gas reacts on the surface of anything it comes in contact with.

The best way to work out during smog is indoors. If you absolutely have to run, the best time is just before dawn. Stay away from traffic as much as you can, head for parks and near a lake or creek.

We really need data from long-term studies before we can know how dangerous smog can be to healthy bodies. People with heart and lung disease need to avoid smog because it causes deaths in these groups each year. If you have allergies or asthma you should also avoid smog.

Some people in great shape and good health think they can handle smog, because the symptoms are often temporary.

Editorial Board Member Ron Lawrence, M.D., Ph.D, says: "From observing Southern California runners for more than 30 years, they seem to have no more lung or heart disease than those who run in cleaner environments. Twenty years ago, we studied 60 runners who ran in traffic, and compared them to non-exercisers. We found lower levels of lead in the runners. We thought this might be due to higher metabolic rate in the runners, which led to a higher turnover of toxic materials."

In spite of this, I'm not convinced we can afford to ignore pollutants. Smog can't do you any good; any effects on your body must be negative.

Twenty years ago I used to run in smog, but now I've read many research papers I don't any more. For the sake of a few days off running outdoors, why take an avoidable risk? Work out indoors on those days.

Copyright, The American Running Association.

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