Air Pollution: The Heat Factor

Heat can increase air pollution in three ways:

1. When weather is hot, primary sources of pollutants tend to increase
2. Sun and heat can transform primary air pollutants into secondary pollutant that can be even more toxic
3. Heatwaves often come with high atmospheric pressure, that creates a stagnant layer of air above ground level. When this happens on cities, air pollutants remain captured, and pollutants density increases

1- Heat increases primary pollutants emissions

Heat triggers more air conditioning power usage in buildings and cars, and this additional power usage emits more air pollution.

Climate change with longer warm periods, can also cause a greater production of plant-based allergens such as pollens (source US White House report).

Other side effects of heat can increase air pollution, such as wild fires that produce high quantities of particles; this pollution can reach densely populated areas, transported by winds.

2- Sun and heat transform - and worsen - air pollution

Sunlight and high temperature trigger chemical reactions between primary air pollutants such as nitrogen oxides (emitted by engines) and oxygen, causing a chemical reaction that forms ozone. The hotter the day and the more intense the sun, the more ozone is formed.
Ozone is a very active oxydant, which exacerbates lung diseases such as asthma and can cause breathing difficulties even in healthy individuals.

Heat and sun also transform primary particles into secondary, smaller particles that can be more toxic.
These secondary particles, which are photochemically produced by sunlight, are of crucial importance: they are ubiquitous and can make up to 90 percent (in number) of the total particulate matter. Secondary particles are smaller than one thousandth of a millimeter (very fine particles) and even smaller (ultra fine particles). They can enter deep into the lung airways when inhaled, and even penetrate the blood.
A team of researchers at the University of Bern (Switzerland) has recently shown that secondary particles from gasoline combustion in engines directly damage lung tissue as well as weaken its defense system as a result of their physical and chemical properties - lung tissue can be damaged, and pathogens (viruses, bacteria) can enter the lungs easier. Asthmatics people are particularly at risk.

3- High atmospheric pressure keeps air pollution at ground level, making its level increase

Heat waves and poor air quality often go hand-in-hand because lingering high pressure creates a stagnant environment. With light winds and no precipitation, pollutants don't get cleared from the air, and they build up right above ground level.
When air quality is poor, people may experiment an acute worsening of their condition, with shortness of breath, a feeling that they can't catch their breath, a tightening in the chest, a cough from the irritation or even bronchitis.