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Hit by a double whammy of exceptionally hot weather and almost no rain, Southern Ontario is experiencing the worst bout of smog that residents of the region with Canada's most notorious air pollution have ever had to endure.

There have been 26 days, covering nearly a third of the summer and including yesterday, during which the air was projected to be so polluted by vehicle exhaust and emissions from smokestacks that breathing was considered a health hazard.

Some researchers are warning the poor air quality in parts of Ontario is approaching U.S. levels, with Toronto starting to have pollution readings more typically found in big U.S. cities, such as New York, where residents receive chronic, long-term exposure to the harmful compounds in dirty air.

"We are probably quite similar to New York City. I would say they are slightly higher, but not that far off" Toronto levels, said Jeffrey Brook, a research scientist at Environment Canada.

Toronto and New York have a heavy pollution burden from the high local use of automobiles and from local industries, he said, but also suffer from the long-range transportation of pollutants from coal-fired electricity-generating stations in the U.S. Midwest.

The smog is being caused by the unusually hot weather, which is projected to continue today, with readings above 30 throughout much of the province. There has been "relentless heat" this summer in Southern Ontario, Environment Canada climate expert David Phillips says.

Since June 1, Toronto has experienced 38 days during which the temperature exceeded 30. In a normal year, there are 15 such days.

As well, there have been 18 nights during which the mercury did not fall below 20, compared to a normal reading of only five such nights.

The hot weather has prompted the extreme air pollution because high temperature causes the emissions from tailpipes and smokestacks to undergo chemical reactions and form smog, Mr. Phillips said.

The number of days projected by the provincial Ministry of Environment this year to have poor air quality has already eclipsed the record of 23 set last year, ministry spokesman John Steele says.

Of the current air-pollution advisory, he said this is the first time one been issued this late in the year.

Normally, cooler temperatures in early September would lead to lower pollution readings than earlier in the summer.

Also, Toronto hasn't had a heavy rainfall since June 21, and has just had its driest August on record, further contributing to the oppressive blanket of smog, Mr. Phillips said.

Normally, rain would wash some of the pollutants out of the air, he said, but that hasn't happened.

Southern Ontario has not been alone in dealing with dirty air.

The high smog readings have frequently extended into Southern Quebec and the Maritimes. Atlantic Canada, where the air is normally clean, received nine advisories this summer, three times the average level.

Environment Canada's Mr. Brook said the air-quality problems of Southern Ontario are the worst in the country, and far more extreme than what is experienced in the most polluted area of the West, around Vancouver and the Lower Fraser Valley.

"We are generally 10 times higher in some of the key pollutants compared to them, even during their [smog episodes], so we're kind of in a different league," he said.