Air pollution exposure contributes to childhood asthma, population-based study shows

New findings from Ontario have shown that children born in Sarnia have a higher risk of developing asthma compared to neighbouring cities.  

A research team from Lawson Health Research Institute and Western University, using provincial data from ICES, found that higher air pollution exposure in the first year of life very likely contributed to this higher risk.  


“It’s known that cities in Southwestern Ontario have varied levels of air pollution because of differences in industry and traffic. For example, Sarnia is home to the ‘Chemical Valley’ where numerous chemical plants and oil refineries are clustered,” says Dr. Dhenuka Radhakrishnan, an Adjunct ICES Scientist, formerly working out of ICES Western in London, and Pediatric Respirologist at CHEO.  

Head shot of Dr. Dhenuka Radhakrishnan
“We wanted to see if children born in three cities – London, Windsor and Sarnia – had a different risk of developing asthma due to the differing air pollution levels in the three regions, even though the people living in these cities are otherwise comparable in many ways.” Dr. Radhakrishnan is a Pediatric Respirologist and Director of the Asthma Program at CHEO, a pediatric health-care and research centre in Ottawa. She is also an Assistant Professor of Pediatrics at the University of Ottawa.  

Asthma is the most common chronic disease in Canadian children and has significant impact on quality of life. Asthma is a leading cause of emergency department visits and hospital admissions in this age group. 

The study followed 114,427 children born in these cities between 1993 and 2009 for 10 years, and found that those in Sarnia were at the highest risk of developing asthma. The researchers found that by the age of 10, nearly 24 per cent of children in Sarnia were diagnosed with asthma, compared to 21 per cent in Windsor and 17 per cent in London. The differences were also present after accounting for many risk factors associated with asthma, such as sex, socioeconomic status and urban versus rural setting. The findings were most apparent in the first two years of life, but persistent beyond the age of six.   

Graph showing proportion of children with asthma diagnosis based on studied region (Sarnia, Windsor and London-Middlesex)

Proportion of children with an asthma diagnosis by studied region

“Reassuringly, we found the asthma risk for children has reduced in more recent years as pollution levels have also decreased,” adds Dr. Radhakrishnan.  

The team asserts that any efforts to advocate for cleaner air would be helpful because air pollution can also trigger asthma attacks in people who already have asthma, as has been previously demonstrated in the Sarnia region and elsewhere.  

Dr. Salimah Shariff
“This study suggests that reducing air pollution exposure, including environmental causes, might reduce the number of children who suffer from asthma and asthma attacks,” explains Dr. Salimah Shariff, Associate Scientist at Lawson, Adjunct Professor at Western and Scientist at ICES Western. 

There is also growing evidence that exposure during pregnancy can influence development of asthma in children.  

“We need to carefully examine how reducing air pollution exposures within a geographic area translates to reductions in asthma development. Understanding the amount of air pollution that a mother and infant are exposed to, and how this impacts their personal risk, could enable regions to target safer levels for their residents,” adds Dr. Shariff. 

Recognizing symptoms of asthma early is the best a parent can do to improve the health of their child, so they can be diagnosed quickly and start appropriate treatments. If a parent notices their child has wheezing, persistent cough or difficulty with exercise, bringing this to the attention of a health care provider is important. 

The study, “Comparing childhood asthma incidence in three neighbouring cities in Southwestern Ontario: a 25-year longitudinal cohort study,” is published in CMAJ Open.  

The research was funded by ICES and completed at the ICES Western site in London, with additional funding from Lawson’s Internal Research Fund.  

Summary of study results:  

  • Children born in Sarnia in the 1990s and early 2000s were disproportionally at a higher risk of developing asthma in the first few years of life, compared to neighbouring cities.  
  • Air pollution exposure in the first year was found to be associated with the development of asthma in children.  
  • Overall rates of new childhood asthma diagnosis in Southwestern Ontario have been decreasing over time in parallel to decreases in air pollution levels. 

Infograph with summary of study results