Asthma rates among children in some parts of North America are four times higher than they were 20 years ago, says a new report that examined links between pollutants and the disease of the airways.

"The air children breathe is an important source of exposure to substances that may potentially harm their health," said the report by the Commission for Environmental Co-operation.

"Exposures in early childhood when the lungs and immune systems are not fully developed raise concerns that children may respond more adversely than adults would."

The Montreal-based commission was set up as part of the North American Free Trade Agreement.

Its report, released Friday, says that in Canada, about 20 per cent of boys and 15 per cent of girls aged eight to 11 have been diagnosed with asthma.

Symptoms of the disease include wheezing, coughing, trouble breathing and chest pain. Relief comes with the use of inhalers or puffers containing drugs to help relax the airways and reduce inflammation.

The report also found that rates of lead contamination and water-related sicknesses have been declining in recent years among children in Canada, the United States and Mexico. However, asthma has become much more common, now affecting about 2.5 million Canadians – including prime minister-designate Stephen Harper, who was diagnosed as a child.

"While heredity plays a role in the development of asthma, it alone cannot adequately explain the large increase in asthma prevalence," the report says.

Other factors include a tendency toward allergies and the presence of pet dander, dust mite antigens, moulds, pesticides, gases or aerosol in the home or school.

However, the report zeroed in on two factors: outside air pollution and smoke in the home from wood or charcoal fires, and second-hand tobacco smoke.

Fewer Canadian children are being exposed to second-hand smoke at home, but the rate is still cause for concern.

About 26 per cent of Canadians 15 to 19 must breathe second-hand smoke in their homes, as are14 per cent of children under age five, according to the report.

Poor urban children 'at greater risk'

As for air pollution, the report said children living in southern Ontario and in poor urban neighbhourhoods are most likely to develop asthma and other breathing problems.

"Lower-income inner-city populations are at greater risk of developing asthma because of sub-optimal levels of care and control, and because they may have higher exposures [to pollutants]," the report said.

The main culprits are ground-level ozone, particulate matter, carbon monoxide, nitrogen dioxide, sulfur dioxide and lead.

"Children are uniquely susceptible and vulnerable to environmental risks – and those risks don't respect boundaries," Herb Gray, Canadian chair of the International Joint Commission of Canada and the United States, said in a statement as the report was released.

"This report will help us protect our children and our environments on both sides of the Canada-U.S. border and in Mexico, too."

Rising asthma rates are placing an extra burden on the health-care systems of all three countries. Asthma is the most common chronic disease afflicting North American children, the report notes, and "is a major cause of child hospitalization."

The report calls for better collection of data on pollutants and other factors linked to asthma, and greater efforts on the part of parents, governments, schools and workplaces to eliminate such triggers.