Along with the lazy and often hazy days of summer come some not-so-pleasant health risks from exposure to air pollution.
Ed Avol, a professor of preventive medicine at the Keck School of Medicine at USC, has been studying the links between respiratory health and air pollution for 35 years. In this edited interview conducted May 19, Avol spoke about the factors that can lead to poor air quality during the summer months and how people can limit their exposure to pollution.
What factors make pollution so severe in the summertime?
Air pollution is kind of a seasonal thing. There are certain kinds of air pollutants that are worse in the summer and certain kinds that are worse in the winter. When we talk about air pollution being worse in the summer, generally we're talking about photochemical oxidants -- things like ozone, things that relate to ultraviolet radiation and sunlight.
You get longer days, more sunlight, more ultraviolet radiation, more stability in the atmosphere -- and so you get stagnant air. That is, the air doesn't move; it sort of just sits and cooks. You have the emissions that come from stationary sources like power plants and boilers, and/or mobile sources like cars, trucks, planes, trains, and ships.
With the sunlight, you get chemistry going on, so new compounds are formed that weren't there before.
Can you describe the main categories of pollutants that are problematic during the summer?
In terms of gases, we often talk about ozone being an issue. That's a photochemical pollutant that historically has been worse in places like Los Angeles and Houston and large urban areas that have a lot of sunlight, a lot of emissions and a lot of stagnation, causing things to cook and become new pollutants.
Ozone is a very reactive gas. It's invisible, so you can't see it, but it's very powerful and potent and it's a respiratory irritant. In the short term, ozone causes chest irritation and a sort of burning-in-the-chest sensation when taking a breath. Ozone is also associated with respiratory illnesses, so we can actually see this in absences from school, for example. In the long term, ozone is associated with a whole range of outcomes including mortality, but also hospital admissions and a number of cardio-respiratory (heart and lung) effects.
There are also a number of other gases that are of concern, such as sulfur dioxide and nitrogen dioxide. These all result from combustion, directly or indirectly. Sulfur dioxide is formed from the impurities of coal or fuel oil. The sulfur in the fuel reacts with oxygen in the air. Nitrogen dioxide is formed from incomplete combustion.
Then there are particles, which can be thought of as little pieces of dirt floating in the air. They are of different sizes, of different chemistry, from different sources, of different shapes, of different biological activity. .
Which sizes of particles pose serious health risks?
The very small particles that we're concerned about in terms of your health are much smaller and typically cannot be seen by the naked eye, but you can see them under a microscope. We talk about them in terms of micrometers in size, or a millionth of a meter. They are typically discussed in the context of what you can breathe or inhale. There are particles of less than 10 microns in size and diameter (PM10), which are inhalable. But those are considered coarse and pretty big particles in terms of what you can breathe in.
Then we talk about particle sizes of 2.5 microns or less (PM2.5), which you breathe into your airways and get into your lungs. And there are very small particles that are less than 0.1 microns in diameter, so small they can actually cross over the air/blood barriers in the lung and get into tissues, travel through the blood, get into cells and disrupt normal cell function.
What's an example of the type of illness you could get from these smallest particles?
It's a hard question to answer. Some of this has to do with what's on the particle. You can think of some particles being coated with chemicals (sort of like a candy coating). Some of this has to do with the physical shape and size of the particle, or where it lands (deposits) in your lungs, where it may cause irritation, inflammation, or swelling.
An example might be if you had asthma, which is a respiratory condition whereby you can get swollen airways, or you might get some excess mucous flow that clogs the airways, making it difficult to breathe – particles may trigger these sorts of events.
Pollution might aggravate a whole range of conditions; for instance in someone with preexisting cardiovascular problems, it might lead to a thickening of the blood vessels through a process of stress and a number of biochemical changes. This can trigger a number of problems in the body, depending again on which organ system you're talking about.
What do researchers now understand about the link between air pollution and new cases of asthma, as well as the role of air pollution in exacerbating existing asthma?