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Weather & Climate Change Wildfires

After the fires: Is the outdoor air pollution really gone?

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By Ziv Lautman on December, 27 2017
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Ziv Lautman, environmental engineer, outdoor enthusiast and BreezoMeter Co-Founder explains why it's important to wait at least 6 days after major forest fires before going outside for extensive activities.

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While there are dozens if not hundreds of different pollutants emitted during a major forest fire, Benzo(a)pyrene is one of the most well-known pollutants. Its effects on our health and how it transforms the environment have been well studied.

The smoke has cleared, but we should still wait to exercise outdoors? Why? What is it about Benzo(a)pyrene that makes it important to wait a few days after a major forest fire before resuming strenuous or extensive outdoor activity?

One of the most important things to remember about indoor and outdoor air pollution, is that we can't always see with our eyes the pollutants that can negatively affect our health.

About Benzo(a)pyrene

Benzo(a)pyrene is a multi-ring aromatic hydrocarbon (PAH), a byproduct of incomplete combustion of organic materials, such as burning of fossil fuels, particularly combustion engines, burning of garbage, and cigarette smoking; grilling of meat and vegetable material; home cooking; and more. Benzo(a)pyrene's natural sources in the environment of are mainly forest fires and volcanic eruptions. Consequently, Benzo(a)pyrene is a relatively common chemical in various environments, found in varying concentrations, depending on the environment and conditions.

Given that its molecular weight is high, and the fact that it is a-polar, Benzo(a)pyrene is hydrophobic, has low solubility and low vapor pressure. The highest concentrations of Benzo(a)pyrene are found in soil, and hence most of the biodegradation of this pollutant occurs with the help of microorganisms in the soil. In water it tends to be attached to sediments and suspended as particulate matter. In the air it is an adjunct to particles, which makes it relatively stable in the atmosphere.

The Pollutant's Half-Life, and Health Concerns

Benzo(a)pyrene's half-life in the atmosphere ranges from one to six days. When attached to particles, Benzo(a)pyrene can be decomposed as a result of photochemical processes and chemical reactions with Ozone, NO2 , SO2 and more.

In animals, Benzo(a)pyrene is considered to be carcinogenic. According to the World Health Organization, it is classified under group A2 - probably carcinogenic in humans. In both humans and animals, Benzo(a)pyrene is readily absorbed in the respiratory tract, mouth, and skin. During the metabolism stages (detoxification by cytochrome P-450), some Benzo(a)pyrene derivatives (especially Benzo[a]pyrene 7,8 diol-9,10- epoxide) react with DNA molecules (Benzo(a)pyrene-DNA coupling), which creates DNA errors and leads to changes during cell replication and to the possible formation of cancer cells. In humans it is found that Benzo(a)pyrene is responsible for a variety of toxic reactions, such as decreased fertility, disruptions in the immune system, damage to red blood cells, and more. For microorganisms Benzo(a)pyrene is not toxic, and is a source of energy and carbon.

Dispersion and Concentration

As mentioned above, most of the Benzo(a)pyrene emissions occur in the atmosphere, but since they tend to be attached to particulate matter, they are transported to distant places and enter water systems and lands in natural sedimentation and rain events. The concentration of Benzo(a)pyrene varies drastically from place to place and depends heavily on anthropogenic activity nearby. The highest concentrations of Benzo(a)pyrene are usually measured in structural spaces such as kitchens during cooking, as well as in coal, oil, and other industrial plants. Despite known decomposition processes, Benzo(a)pyrene is considered to be a highly stable pollutant (or POP – Persistent Organic Pollutant), mainly due to its tendency to be attached to various particles.

Due to its high stability in the environment and up to six day half life, it might be a good idea to wait 6 days or more after a major forest fire, before doing extensive outdoor activity, just to be on the safe side. Moreover, remember that Benzo(a)pyrene is not being regularly measured by environmental agencies and information about its existence in the atmosphere is usually not available.

Read more about how wild fires affect air quality.

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