It’s Not the Heat, It’s the Humidity… and the Pollution

In fact, all three contribute to increased mortality and hospital visits, particularly among the elderly. But it's not just during "heat wave" events.


In recent years, an alarming increase in heat waves and poor air quality has captured the attention of people worldwide. The implications of this trend have been profound, as evidenced by studies highlighting a concerning correlation between extreme heat events and heat-related mortalities and hospital visits.

This alarming pattern has been observed not only in hotter regions of the world such as India and the southern United States but also within Canada.

However, it’s not just the most extreme heat events that pose a danger. A new wave of research has shed light on the fact that even non-extreme heat conditions, characterized by temperatures in the humidex range of 30 to 35, have been associated with increased mortality rates — especially among the elderly. This unexpected finding prompted a deeper dive into the intricate relationship between heat, air quality, and human health.

To investigate this further, Dr. Mohamed Dardir a postdoctoral researcher in the School of Environment, Enterprise, and Development at the University of Waterloo, and colleagues embarked on a mission to uncover the short-term impacts of hot climate conditions intertwined with poor air quality on community health records such as mortality rates and emergency department visits. The study was published in Environmental Research in January of this year.

Measurements of hot climate conditions were based on ambient air temperature, relative humidity, and solar radiation, while poor air quality was based on the concentrations of ground-level ozone (O3) and fine particulate matter (PM2.5) in the air from May to September 2003 to 2017. The areas of interest included Mississauga and Brampton, two cities in Ontario.

The study found a positive correlation between deaths and emergency visits with hot climate conditions and air pollution, but the effects were more pronounced two days following the environmental event. Strikingly, it was moderate heat conditions — not extreme ones — that had a greater association with increased mortality and emergency visits among the elderly (older than 65 years) and those with cardiorespiratory problems.

Moreover, the study highlights an unsettling trend: a mere 10°C increase in maximum temperature was linked to around a 1% rise in the probability of cardiorespiratory deaths among the elderly. Similar patterns emerged for increases in PM2.5 and O3 concentrations in the air for elderly individuals with cardiorespiratory problems.

In short, this research shows that when air pollution and high temperatures increase in some areas of the GTA, there’s a higher chance of more people, especially elderly individuals with heart and lung problems, getting seriously ill or even dying.

The study suggests practical steps that public health officials can take to better protect people during these severe environmental conditions. It also emphasizes how important it is to deal with climate change: not just for the planet’s sake, but for our own health as well.

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Alexandria (Alex) Samson is a PhD student in the Department of Psychology at the University of Toronto. She completed her BSc in Neuroscience from Dalhousie University. Alex is a strong believer in open science and is passionate about making scientific research accessible to all audiences.