In old movies set in London, you often see thick, swirling fog obscuring the quaint old city streets, turning everything mysterious, threatening, or romantic -- depending on the movie.

London's "pea-soup" fog was a boon to movie-makers but did little to benefit Londoners. A major component of the swirling fog or, more properly, smog was fine particles spewed out of chimneys from dirty coal fires.

Decades ago, researchers found a direct relationship between the Total Suspended Particulates (TSP) in London fog and human death rates. The death rate -- from all causes, not just lung disease -- climbed significantly whenever the smog thickened and lingered.

Restrictions on the burning of coal have made London pea-soup fogs largely a thing of the past. However, later research has established that coal is not the only culprit.

Anything that loads the air with fine particles is a threat to human health, says Dr. Val Cunningham.

Cunningham has been a member of both the Yukon Medical Association's Health Promotions Committee and Whitehorse's Woodsmoke Committee. In the Yukon, the most common source of airborne particles in winter is woodsmoke.

Fine particles, less than 10 micrometres in diameter, are the most dangerous, she says.

"They can traverse walls or move anywhere air is moving," says Cunningham. "The finer the particle, the deeper it gets into your lungs."

Researchers still haven't established exactly how the tiny particles damage the lungs or why their presence seems to have such broad health effects, but the pattern of health effects is clear.

Recent studies in locations that regularly experience high levels of suspended particles in the air also looked at other factors, such as temperature and humidity, that might have an impact on human health. The only factor that showed a clear and direct relationship with health effects was the particle level, Cunningham says.

The health effects can be dramatic. Studies have documented a link between poor air quality and a higher death rate from lung and heart disease, increased emergency room use, more asthma attacks, and increased absenteeism from school.

Despite the Yukon's remoteness from large industrial and population centres, it is not safe from the dangers of particle-laden air.

"Whitehorse is one of the worst areas in Canada for woodsmoke, because it's in the valley and there's nowhere for the smoke to go," says Cunningham.

In the past, the City of Whitehorse used a set of instruments mounted on a 25-metre tower to monitor woodsmoke levels in its Riverdale subdivision. When the Total Suspended Particulate (TSP) level reached 110 micrograms per cubic metre, the City declared a No Burn Period, requiring Riverdale residents to switch to other forms of home heating.

Cunningham says even that TSP level might have been too high for safety.

"The toxic level is arbitrary," she says. "It's what you're prepared to accept."

Based on current data, the level of smoke that used to trigger No Burn Periods in Riverdale is commonly associated with a 24 percent increase in school absenteeism, 17 percent increase in asthmatic attacks, and 19 percent increase in death from respiratory causes. Lowering the impact on health would mean lowering the trigger level for instituting a No Burn Period, Cunningham says.

Currently any discussion of lowering the trigger level is purely hypothetical, since the City of Whitehorse has decided not to activate the instruments on the woodsmoke monitoring tower this winter.

"There's no way of triggering a No Burn Period at present," says Cunningham.

For more information, contact Environment Canada, Whitehorse.