The Great Smog gripped the city of London for four days in 1952.  Smog was not an unusual occurrence in London, but this one was different. It started after a prolonged cold front caused coal fireplaces to work overtime to heat homes. Next, a high-pressure system caused warm air to lie over the cooler ground air. These events prevented the smoke generated from burning coal from rising and escaping.  The smog crippled London, paralyzing transportation. Londoners could not see their feet as they walked and their faces were black with soot if they ventured outside. The Great Smog was deadly as well with approximately 4000 deaths directly attributed to the smog. About half of the deaths were attributed to respiratory disease, but about 25% were due to heart disease.  Due to the Great Smog, Parliament passed the Clean Air Act of 1956, restricting coal use in cities and switching to gas, oil and electricity for heating. In the US in the 1940’s and 1950’s, Pittsburgh symbolized the evils of air pollution. The smog from the steel mills was so thick that street lights had to be turned on during the day. In 1948, Donora Pennsylvania, a town near Pittsburgh, was hit with thick yellow smog that resulted in 20 sudden deaths, 400 hospitalizations and caused 7000 of the 14000 residents to become ill.  As a result, Congress passed the Clean Air Act of 1963 and the Air Quality Act of 1967, both of which helped limit the unhealthy effect of air pollution on the US population.  Air pollution has been associated with heart disease for many years. How and why does air pollution affect the heart?

The unhealthy effects of air pollution are caused by inhaling fine particles in the air. These fine particles can be produced by natural sources, such as dust and wildfires. They are also caused by industrial activity and burning fuel in homes. In addition, a major contributor is emissions from cars and airplanes.  Inhaling these fine particles from the air can stimulate localized and more widespread reactions in the body. These fine particles provoke inflammation in the lungs and the inflammatory cells and proteins produced are released into the body, causing a systemic inflammatory response. Levels of C Reactive Protein, a measure of inflammation, are elevated in the blood after exposure to air pollution.  Local inflammation can trigger asthma and emphysema in the lungs. Exposure to the particles in air pollution has been shown to increase the risk for heart artery disease and increase blood pressure. Since plaque within the heart arteries is an inflammatory process, anything that increases inflammation can increase the severity of blockage in the coronary arteries. This was shown in a ten-year study of calcium in the heart arteries (a marker for plaque in the arteries).  Patients exposed to high levels of traffic-related air pollution in cities had progressive worsening of the calcium in their arteries.  In other words, constant exposure to air pollution accelerates atherosclerosis in the heart arteries. Lastly, air pollutants can increase the risk of blood clotting within the body. All of these factors, elevated blood pressure, the systemic inflammatory response and the increased the risk for blood clotting are mechanisms whereby air pollution can destabilize plaque and trigger a heart attack.  The association between long term exposure to air pollution and heart attacks has been corroborated in many studies worldwide. In addition, exposure to air pollutants has been shown to increase admissions for congestive heart failure and to increase the risk for atrial fibrillation.  Those at higher risk for air pollution related heart disease include people with pre-existing heart disease, those with diabetes and the elderly. 

Given the global burden of disease caused by air pollution and the fact that it is a leading modifiable risk factor for heart disease, how can the effects of air pollution be reduced? On an individual level, those at risk should: 1) travel by walking or public transportation rather than by car; 2) avoid being outside in rush hour traffic; 3) exercise in parks and avoid major traffic roads; 4) limit time outdoors during highly polluted periods and 5) use a ventilation system with filtration for homes in high pollution areas.  On the global level, countries should work together to reduce industrial pollution, stop the rise in the global average temperature, reduce emissions and switch from fossil fuels to greener sources of energy.  Signing on to the Paris climate agreement would be a good first step.

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