When we step outside on a summer afternoon, we can often feel the dense, stifling quality of the air. In winter, the stillness can feel even thicker, making us feel like we’re choking on air. Most of us assume that air pollution is at its worst on scorching days, but in reality, extreme temperatures, whether high or low, greatly shape the air around us. So, how do hot and cold temperatures influence air pollution levels, and what does it mean for our health?

Changing Seasons

Air pollution changes hourly, daily and also throughout the year, with summer and winter each bringing distinct kinds of environmental stress that can result in potential air quality deterioration.

In winter, air pollution is often at its most intense. One reason is the denseness of cold air, which traps pollution close to the ground. During a cold night, the ground cools faster than the air above it, forming a lid that keeps pollutants, like nitrogen dioxide (NO₂) and fine particulate matter (PM2.5), from dispersing. This phenomenon is known as a temperature inversion.

During nighttime temperature inversion episodes, levels of fine particulate matter increase by approximately 54%, while nitrogen dioxide increases by 49% compared to non-inversion periods; the cold traps these harmful particles and keeps them in the range of people.

In contrast, summer brings a different kind of threat: ozone pollution. Ground-level ozone forms when nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) react under sunlight, which is accelerated during hot weather, leading to elevated ozone levels that inflame airways, aggravate asthma, and increase the risk of cardiovascular problems. This ozone pollution is what leads to the dusty summer haze that we are all so familiar with; it especially settles over urban centres, where pollutants and emissions are at their peak.

In addition, in recent years, wildfires, which are intensified by climate change, heatwaves and drought, have added another layer to summer pollution. Smoke from wildfires sends clouds of PM2.5 across entire regions, causing severe health impacts even in places far from the flames. In some cases, wildfire smoke has traveled thousands of kilometers, triggering air quality worries across continents.

Temperature and pollution work hand in hand, and the combination of the two can have dire consequences. While extreme heat alone increases mortality risk by 6.1% and extreme pollution alone raises it by 5%, the two occurring together makes the risk surge to 21%, which is more than the sum of each factor alone. High temperatures make the cardiovascular and respiratory systems work harder, and pollutants like ozone and PM2.5 inflame airways and thicken the blood. Together, they impair the body’s ability to cool itself and maintain stable function. For vulnerable groups, like the elderly, children, or people with asthma, this creates an abundance of physical strain that is difficult to fight against.

Weather and daily life

But temperature doesn't just shape pollution; it affects how we experience it. In moderate warmth, people tend to spend more time outside, increasing their exposure to ozone and PM. But when it gets too hot or too cold, many seek shelter indoors if they can, offering them a better shield from poor air. Even so, for people without access to clean indoor air, these extreme conditions can become life-threatening.

Seasonal shifts also influence pollution sources. Summer often brings more barbecues, construction, and even fireworks, all of which contribute to VOCs and particulate matter. In winter, heating sources like wood stoves and coal furnaces become the main contributors, especially in areas lacking access to clean energy alternatives. Plus, during the winter, there is an increase in gas usage for transportation; it is too cold outside to walk, run, or cycle, and more people are using cars to get around, which releases higher carbon emissions.

Poor air-quality disproportionately affects minorities too; in low- and middle-income countries, women are often at higher risk of pollution exposure due to traditional roles that involve cooking and heating with solid fuels. This exposure worsens in colder seasons when indoor energy use increases and ventilation decreases.

Health risks and temperature extremes

But whether it is hot or cold, temperature extremes paired with polluted air have compounding effects on human health. In winter, the body is already working to regulate internal temperature, and high PM or NO₂ exposure on top of this increases risks of heart attacks, strokes, and respiratory infections. Meanwhile, in summer, ozone and heat together raise risks of asthma attacks, heat stroke, and cardiovascular issues.

A King’s College London study reported that in nine major English cities (including London, Birmingham and Manchester), high air pollution days would lead to 124 more cardiac arrests, 231 strokes, and 193 asthma-related hospital admissions annually, compared to cleaner days

These dangers are prompting changes. For example, the 2024 Paris Olympic marathon was rescheduled to take place at night to avoid the concerning combination of midday heat and ozone pollution.

Future-focused solutions

But how else can we take measures to protect ourselves? Key initiatives may look like using clean air alternatives to coal and wood in the winter, as well as improving home insulation and indoor air filtration. They may look like cutting back on pyrotechnics and encouraging people to wear masks during wildfire seasons. Meanwhile, the transition to electric vehicles significantly reduces fossil fuel burning, and in tandem, NO2 and ground-level ozone; each of these solutions can help us keep cleaner lungs.

Air Aware Labs and monitoring the extremes

At Air Aware Labs, we recognise that clean air is a year-round issue. Through our AirTrack app, we help people track pollutants like NO2 and PM2.5 each season; understanding how heat and cold worsen air pollution is key to protecting public health, especially as climate change intensifies these extremes. By preparing for temperature-based pollution changes, we can build more breathable cities.

By Annika Budhwani overseen by Dr Will Hicks

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