Air Pollution: The Heat Factor

Mortality increase due to heat waves has been extensively studied. For example, it is estimated that 70,000 additional deaths occurred during 2003 heatwave in Europe. The increasing frequency and intensity of heatwaves projected for the coming century is an important health concern.

Air pollution impact on health is another important issue. The World Health Organization estimated that in 2012, 7 millions premature deaths were caused by air pollution, worldwide.

Heat waves threaten health. Air pollution threatens health. But why do alerts on heat and air pollution often come together?

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
Heat waves threaten health

Let’s now dive into these three-fold impacts.

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.

Ultrafine particles are so small and lightweight that they are a negligible share of the renowned PM2.5 measurement. However, they are a critical factor of the air pollution.

"Ultrafine particles represent 80% of the particles in the air of cities", says Francelyne Marano, Professor in Biology at the Paris-Diderot University. "But we have no technology to accurately measure them. That’s why there is currently no regulation for ultrafine particles."

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.

Finally, Global Warming and air pollution combine into a major public health concern.

In the past three decades, the percentage of Americans with asthma has more than doubled, and climate change is putting those Americans at greater risk of landing in the hospital. Already, more than 8 percent of Americans are living with asthma, including more than 9% of children. In fact, asthma is the third leading cause of hospitalizations for children.

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