A cyclist rides along a path near the Parliament buildings in Ottawa during a heat and smog alert. (Jonathan Hayward/Canadian Press)

It's another hot, smoggy day in the city and you're desperate for a run.

Yet every time you read the news or listen to the radio, there's some health expert telling you about the dangers of mixing exercise and air pollution on a hot, muggy day.

The problem - as those same health experts will tell you — is that exercise is crucial to your well-being.

While this contradiction may not have been a big deal before the days of frequent smog and heat alerts, it's becoming a real headache for the health-conscious and for high-level athletes.

World-class Canadian triathlete Lisa Bentley has eleven Ironman wins under her belt, but she didn't get there by avoiding exercise on hot, smoggy days.

"Training is my career, it's my job, it's what I do, and I have to go to work every day. So if I go to work in the smog, I go to work in the smog.

"I love what I do too much to let the poor air quality keep me back from it …Unfortunately I will take the risks that go with that," Bentley said.

But public health officials worry that heat and smog alerts, while designed to protect the public, may inadvertently discourage others who should be more active.

And the best time to exercise is?

Recognizing the dilemma, Toronto Public Health and Health Canada studied the issue to identify convenient times of the day when it's safer for vigorous outdoor exercise.

Instead, researchers found that for most people, there really is no convenient time to go for that jog.

"In Toronto, pollution levels tend to be lowest overall … before 7 a.m. in the morning and after 8 p.m. in the evening," public health officials wrote.

The study measured hourly fluctuations of air pollution in Canada's largest city over a four-year period.

It found that carbon monoxide and nitrogen dioxide are at their highest levels during rush hour. The levels of these two pollutants, the product of car exhaust, begin rising at 6 a.m. and peak at 8 a.m.

The author takes a walk break and douses himself with water in the final kilometres of the Athens marathon in November 2003, where the temperature hovered around 30 C. (CBC)Ozone levels meanwhile peak in the afternoon after sunlight has had a chance to "cook" a mixture of air-born nitrogen oxides and volatile organic compounds. Sulphur dioxide also peaks just after mid-day.

And levels of particulate matter, those tiny bits of pollution that get caught in your lungs, are high all day long during smog alerts.

"There's no time of the day when all pollutants together are at their lowest level, other than the deep dark night. And that's not very helpful," notes Monica Campbell, one of the report's authors and a medical toxicologist with the city of Toronto.

So unless you can exercise before 7 a.m. or after 8 p.m., your best bet is to head indoors, she says.

Toronto Rehab respirologist Michael Sarin agrees. "Air-conditioned buildings have filters [that trap pollution] and ozone doesn't penetrate inside buildings," says Sarin, who recommends his patients take a brisk walk through the mall on smog advisory days.

Sarin says the problem with outdoor exercise is that we breathe harder and take in more pollutants.

"The pollution by-passes the nose, which filters air pollution, and gets directly into the bronchial tubes, which causes deeper injury," he says.

Ozone penetrates bronchial tissue, sensitizing it so that other pollutants can then inflame the tissue, making it harder to breathe.

That's why it's important to know when you're at greatest risk for outdoor air pollution. Sarin's colleague at Toronto Rehab, cardio rehab supervisor Kerseri Naidoo, recommends everyone, not just those with respiratory or cardiac problems, understand the difference between a smog watch and an advisory.

A smog watch is issued when there is at least a 50 per cent probability of smog within the next three days. A smog advisory is issued when there is a high probability of elevated smog levels within the next 24 hours, or if smog conditions happen without warning.

But if despite high air pollution levels, you decide to go for that run anyway, you might like to know that for healthy people, the benefits of exercise probably outweigh the risks of exposure to heat and smog.

Health experts says if you are going to exercise outdoors in hot and humid conditions, you need to drink plenty of water, take frequent rests and pay attention to how your body feels.

Problems with breathing, coughing, chest pain, nausea, headaches and weakness are your body's way of telling you to take it easy. If you experience any of the signs of heat sickness — such as nausea, blurry vision and dizziness — while exercising, stop immediately. Find a shady or cool place and drink some water. If your symptoms persist, seek medical help.

When it's hot out and you're working out, reduce your expectations. If you're running or cycling, ease up on your pace. Don't be disappointed when you can't maintain the level of exercise you normally do when the weather's cooler.

And try to exercise in parks or along low-traffic residential roads, which will keep you away from car exhaust.

Endurance athletes aren't the only ones who should be concerned about the conditions. Golfers and recreational softball players should pay close attention as well. You will sweat if you're spending a couple of hours moving around on a hot day — and that means your body is losing vitally important fluids that you will have to replace.

You can celebrate that scorching day on the links or the ball diamond — or surviving that workout in the heat — with a beer later on. Just make sure you have plenty of water first.