Now that we are in the “dog days” of summer, many of us hear reports on TV or the radio regarding air quality. Because these warnings are usually directed towards people with lung disease or heart disease, I think it’s important to understand what’s behind the “air quality” ratings.
In general, the “air quality” refers to the ozone level. What is ozone? Ozone (chemical symbol: O3) is a molecule that occurs as a result of the interaction between sunlight and pollutants. Most of these pollutants are products of the combustion of fossil fuels, so it is not surprising that the ozone level in the air is highest near highways, factories, and power plants, and tend to be highest where the are large population centers, such as California and the East Coast of the USA. Ozone forms when molecular oxygen (O2), which is vital to life, interacts with nitrogen and other chemical compounds to form O3. Because this chemical reaction is driven by sunlight, it is also not surprising that ozone levels are higher in the late spring and summer months, and also from the mid-morning (right after rush hour!) until the evening.
It is also important to note that this ozone situation is different from the problem of depletion of the ozone layer. When we talk about the ozone layer that protects the planet Earth from dangerous radiation from the sun, we are referring to the ozone layer that occupies the stratosphere. When we discuss the ozone that causes health problems as a principal component of smog, we are referring to the level of ozone in the troposphere, or the layer of air that we encounter in our daily lives.
Ozone is known to cause lung inflammation. In animals exposed to high levels of ozone, the lungs become filled with a variety of inflammatory cells and their chemical products. In humans, chemical markers of inflammation also increase after ozone exposure. Symptoms such as chest pain or tightness, decreased lung function, and cough. There is evidence that individuals with asthma may experience increased sensitivity after they are exposed to ozone. People who exercise seem to be more susceptible to ozone, with symptoms occurring at lower levels of ozone. Repeat exposure to high ozone levels may result in persistent lung inflammation. People who live for a long time in regions where ozone levels are high seem to experience small decreases in lung function.
Asthma and COPD appear to be aggravated by high ozone levels and the frequency of visits to the Emergency Department for respiratory problems increases during periods when the ozone levels are high. Worse, the death rates from lung disease and cardiovascular disease also tend to increase during periods when the ozone levels are high.
Recently, the Environmental Protection Agency (EPA) tightened the air quality standards, lowering the amount of ozone permitted in the air from 84 parts-per-billion to 75 parts-per-billion. Although this seems like a positive step, many criticized the EPA for not lowering the limits enough: the EPA’s board of scientific advisors had suggested lowering the limit to 60 or 70 parts-per-billion. Worse still, many areas cannot even achieve the old limit of 84 parts-per-billion. Achieving these lower levels is difficult and expensive.