Should You Skip Your Walk When The Air Is Bad?

You've been told to walk every day. Thirty minutes, rain or shine.

But some mornings the sky looks brown at the edge of town. Your weather app turns orange. And you wonder if each deep breath on your walk does more harm than good.

It's a fair question. For once, there's real data on both sides of it…

In October 2012, a team at Imperial College London began a simple test. They asked 119 people aged 60 and over to take a two-hour walk on two different days.

One walk ran along the west end of Oxford Street. That's London's busiest shopping street, packed with diesel buses and cabs. The other walk ran through Hyde Park. That route is quiet and car-free.

Of the walkers, 40 were healthy. Another 40 had a long-term lung disease, and 39 had heart disease. Rudy Sinharay, a lung doctor, ran the walks, and Kian Fan Chung led the team. The British Heart Foundation paid for the work.

The results ran in The Lancet in 2018…

What Two Hours On Oxford Street Did

In the park, the walk did what walks do. People's lungs moved more air for hours afterward. Their arteries got less stiff, and stayed that way for up to 26 hours.

On Oxford Street, most of that gain shrank or vanished. Fine particles, soot and a traffic gas called nitrogen dioxide were all higher there than in the park. Walkers with lung disease also coughed more after the street walk.

So should you stay home on bad days?

Not so fast. That test looked at one walk and the day after it. The bigger question is what happens over years…

The People Who Were Followed For Years

Zorana Jovanovic Andersen at the University of Copenhagen took that question to 52,061 Danes aged 50 to 65. They lived in Copenhagen and Aarhus, and her team tracked them from the 1990s to 2010.

She expected the traffic fumes to eat into the gains from exercise. That did not happen. They didn't. People who were active had about 20 percent fewer deaths than those who weren't, even in the most polluted parts of town. Sports, cycling and gardening all showed the link, though walking alone did not reach a clear result.

A University of Hong Kong team found the same pattern in 66,820 adults aged 65 and over, in clean and dirty parts of the city alike. That one came out in 2020.

Both studies were observational. They watched people rather than assigning anything, and they measured the air at home, not on the walk.

The Study That Gives Me Pause

Now the other side. Seong Rae Kim at Seoul National University looked at 1,469,972 South Koreans aged 20 to 39. Each had two health checks between 2009 and 2012, and the team tracked heart and blood vessel disease through 2018.

Where the air was clean or middling, people who went from no exercise to a solid weekly dose had 27 percent less of that disease. In areas where fine particles ran high, the same jump came with 33 percent more. It ran in the European Heart Journal in 2021.

Hold that second number loosely. It fell just short of the usual bar for a sure result, so it could be chance. The team also had no way to tell whether people worked out indoors or outside.

And "high" in that study meant about 26 micrograms of fine particles in each cubic meter of air, or more. That's only a little above the world average in the Cambridge model.

Here's what I believe. The Korean result doesn't say stop moving. It says the hardest workouts on the dirtiest days are the ones to rethink…

What A Mask Showed, And Didn't

In a 2009 study, Jeremy Langrish at the University of Edinburgh sent 15 healthy young people on a two-hour walk through central Beijing. With a tight dust mask on, their top blood pressure number averaged 114, against 121 without it.

That's one small study. The walkers knew when they were breathing filtered air, which can sway a result. Nobody has shown that a mask on your daily walk changes your long-term health.

Where Your Plate Fits In

Fresh air has always sat right next to movement among the 7 principles of health. The old idea was simple: get outside, breathe deep, use your legs. Modern streets can put those two at odds.

Can food help? One trial hints at it. Patricia Egner and Thomas Kensler at Johns Hopkins gave 291 adults in Qidong, a farm area in China with heavy air pollution, a daily drink for 12 weeks. Half the drinks held broccoli sprout powder, and the rest were plain juice.

The sprout drinkers flushed out 61 percent more of a marker for benzene in their urine. Benzene is a gas in traffic fumes. A marker for acrolein, another fume chemical, rose 23 percent. The trial ran in Cancer Prevention Research in 2014.

But that powder was made strong for the trial, and the test measured what left the body, not who got sick. Nobody has tested whether eating broccoli sprouts protects your heart or lungs on a bad-air day. I'd eat them because they're good food, not as a shield…

Where The Evidence Runs Thin

Let me be plain about the gaps.

No trial has assigned people to exercise in dirty air for years and then counted heart attacks. Almost every long study measured pollution at home, not along the route. And the best data come from Denmark, Hong Kong and South Korea, not from the most polluted cities on earth.

So the honest answer has two halves. Over the long run, moving beats sitting in almost every city studied. On a single bad day, where you move and how hard you push both matter.

Three Things Worth Knowing Before You Head Out

Those 119 walkers in London didn't prove that a city walk is bad for you. They showed that the same two hours did more for the body in a park than beside the buses.

And the Danes and Hong Kong elders who kept moving still came out ahead.

So check the air this week. Then take the back street, and go.

Stay well,
The Healing Plate