Study Questions Link Between Air Pollution, Serious Health Effects

March 18, 2010 - News Release

Challenging accepted wisdom, a University of Guelph professor says claims about the health effects of air pollution are not supported by data from Canadian cities.

Guelph economist Ross McKitrick, along with Gary Koop of the University of Strathclyde in Glasgow and Lise Tole of the University of Edinburgh, analyzed a new database from 11 Canadian cities over a 20-year period. Unlike most earlier studies, this one included controls for effects of smoking and income.

They found no evidence that air pollution affected either hospital admission rates or time spent in hospitals. However, they did determine that both smoking and income levels directly affect respiratory health. Their findings appear this week in the journal Environmental Modelling and Software.

The researchers compared monthly hospital admission rates between 1974 and 1994 for all lung ailments to ambient levels of five common air contaminants. “We were looking for predictable, common physical effects from standardized exposure levels,” McKitrick said, adding the researchers examined data over a longer time span than most previous studies, and used advanced econometric methods called Bayesian Model Averaging to ensure they considered all possible combinations of effects.

“Our examination of data back to the early 1970s was motivated in part by the fact that air pollution was much higher compared to today,” he said. “If today’s air pollution levels are causing thousands of hospitalizations, the effects should have been even stronger in the 1970s when air quality was much worse.”

“But the data showed no evidence of changing health effects at the pollution levels observed in Canada over recent decades.”

The findings contradict hundreds of studies that have connected urban air pollution levels and respiratory health problems. Such studies have resulted in calls for tighter air pollution regulations and more stringent emission standards.

McKitrick said the discrepancies between this study and earlier research stem from the common practice of not examining long enough data sets and not controlling for model uncertainty, smoking rates and socioeconomic variables. He added that their study drew data samples from the 1970s, when many Canadian cities had high pollution levels, through the 1980s, when steady reductions began, and into the 1990s, when pollution levels were historically low.

“It’s important to get accurate measures of the potential benefits of air pollution regulations, namely improved quality of life and reduced health-care costs, in order to guide regulatory decision-making,” McKitrick said.

“We did find consistent evidence that lower smoking rates lead to fewer hospital admissions and shorter stays,” he said. The researchers also found evidence that, all else being equal, regions with larger economies tend to have higher hospital admission rates. This may indicate more hospitals and longer patient treatment regimens, McKitrick said.

Prof. Ross McKitrick
Department of Economics
519 824-4120, Ext. 52532

For media questions, Communications and Public Affairs: Lori Bona Hunt, Ext. 53338, or Deirdre Healey, Ext. 56982,

University of Guelph
50 Stone Road East
Guelph, Ontario, N1G 2W1