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?
In between episodes, an asthmatic can breathe and be just like their peers. But during an asthmatic event, they often describe the sensation as being like someone standing on their chest, and they can’t get a breath. Their airways swell up. The picture you might think about is of trying to breathe through a straw. You can’t get enough air into your lungs, and it’s very frightening. For some it can be life-threatening. And air pollution appears to be one of the things that can trigger an asthmatic event.
Studies we’ve done in California show that among children who have asthma and breathe the higher particulate pollution, they tend to have more bronchitis, more symptoms, more breathing problems.
We’ve also done some studies and shown that children exercising in areas of higher ozone are at higher risk of developing the disease, so there actually seems to be some causal effects of air pollution.
One thing that is clear is that people are at higher risk for having asthma and for having a triggered event when they live closer to busy roadways. There’s something associated with busy roadways and traffic that we think makes it worse.
Can you discuss the role that air pollution plays in cardiovascular disease and other non-respiratory illnesses?
It’s not so much that you breathe air pollution into your heart, although it does turns out that some very small particles have been found in the bloodstream and the heart muscles. But there are secondary or tertiary effects from breathing in particles. When you breathe them in, it leads to inflammation which may trigger some disruption or change in normal cellular function; in turn this may lead to some other problem that ultimately creates a response in the body.
With cardiovascular disease, your heart has to work harder to get oxygen to the tissues, or the blood vessels get more clogged and you can develop this atherosclerotic plaque formation. These are the kinds of effects that have been associated with air pollution exposure.
With each passing year and additional study, we’re also seeing air pollution being associated with other things such as low birth weight and birth defects; it’s even been suggested recently that there are links between increased autism and proximity to roadways. So there’s a whole range of respiratory and cardiovascular and neurological problems associated with long-term exposure.
In these sorts of population-based studies, there’s rarely a smoking gun where you can say ‘here’s a cause and here’s an effect’. What we can say is that there are associations, there are increased risks, there’s something going on that makes us want to look closer.
What makes some people more susceptible than others?
Some of it has to do with genetics -- you have certain types of genes, or you don’t, that help your body process or not process certain chemicals better than others.
We are concerned particularly about children, because children have developing tissues; they’re not fully formed, they’re not fully-grown, and we know that biological systems when they’re in development can be very sensitive. So we need to be thoughtful about when and where they might be exposed to air pollution, such as when they exercise.
We know that people with pre-existing disease, such as respiratory and cardiovascular problems, are sensitive. Pregnant mothers are also a sensitive group.
Surprisingly, it turns out that athletes are also a sensitive group. You wonder why that should be since these are very healthy, robust individuals. The reason why they are sensitive is because they are very determined and committed to their training schedules, and they exercise every day for long periods at high rates of ventilation, whether it’s hot, smoggy, clean; it doesn’t matter, they go out and they train. If you think of your lungs as a big vacuum cleaner, for example, if you’re running much harder, you’re breathing much faster, you’re taking much more air into your body.
The elderly are also susceptible because they have some compromised systems. They may not have that extra capacity or extra breathing volume, and a little bit of stress may be enough to cause them trouble.
What can people due to limit their exposure?
Think about where and when you exercise. You may say, ‘Well, I live in a city, I’m not fortunate enough to be able to run down a country road. What do I do?’ There are some things you can do. Don’t run on the main highway; run several blocks back in the neighborhood. Don’t run at rush hour; run at other times of day such as in the evening or early morning. Of course, you also want to keep safety issues in mind.
Having said that, we do want to emphasize that exercise is a good thing in terms of your heart health, your psychological health, and your physical health. So you just need to make smart decisions about where and when and how you exercise. You may not be able to see air pollution, but it may still be there.
Earlier in the Industrial Revolution, people could see dirty air because it was smoky, it was black, it was foul, and so people more readily believed that it was bad to breathe. Now the air is really, truly cleaner than it has been, and it’s getting cleaner. There’s a lot of great progress that’s been made in terms of controls and cleaner fuels and cars. But there’s still a ways to go.
The problem is that now often the particles and gases are of a concentration or in a size that you can’t see; the gases might be clear or the pollution levels might be low so you think it’s OK, but it may or may not be.