The (indoor) air that we breathe

This time of year, with houses closed up for so long because of air-conditioning, many people long for a cool and breezy day so they can throw open the windows and air the place out.

The urge to air, it turns out, is based in science. Research has shown that, in some instances, indoor air can be more polluted than outdoor air.

The Indoor Environment Research Group at Drexel University, led by associate professor Michael S. Waring, has focused much of its research on our indoor air. We spoke to him recently about concerns and solutions.

Tell us about indoor air pollution in general. How big a problem is it?

You can think of a home as this little chemical reactor stuck in the massive outdoor fluid, which is the outdoor air. Some of that air comes inside the house. More emissions and chemical transformations go on indoors. There are thousands of pollutants in the outdoor and indoor air.

Since people spend 90 percent of their time indoors, most exposure to airborne pollutants occurs indoors. Even if the pollution comes from the outdoors. One example is fine particulate matter, often an outdoor-emitted pollutant. But it also is generated indoors due to things like cooking. Cooking can increase particle concentrations indoors up to 10 to 100 times more than what they are outdoors. Any other unvented combustion — candles, space heaters — also will make particles. Cleaning solutions will make particles, due to chemical reactions with ozone.

So, how big of a problem is it? In a country like the U.S., where we have strict building standards and relatively clean environments for most people, it’s hard to say how big a health problem this is. What we know about air pollution and health is largely from epidemiological studies that correlate changes in outdoor pollution concentrations with changes in health impacts. We can apply what we know about outdoor concentrations to indoor pollution levels, but that’s somewhat spurious because indoor pollutants are of somewhat different types. And this ignores all the pollution generated indoors.

What we do know is that certain classes of pollutants give certain responses in people. We know that particle exposure can lead to premature morbidity and mortality. We breathe the particles in, and the smallest of them penetrate our lungs deeply. We know that exposure to formaldehyde, which is used in some furniture, can cause cancer. We know that exposure to chloroform, which is off-gassed from municipal drinking water, can cause central nervous system depression. We know that exposure to nitrogen dioxide, which is emitted from combustion processes such as gas stove use, is associated with asthma.

Assuming you don’t live near a source of significant outdoor air pollution, is it a good idea to open the windows and air out your home?

Sure. Increased ventilation with clean air is always a good thing. We tend to keep our homes pretty tight — especially in very cold or warm seasons — which is good for reducing energy use, but what we’re doing is keeping indoor-generated pollutants indoors longer.

But ventilation won’t fix everything. Carpeting is a primary example. It has a lot of surface area and acts almost like a chemical sponge for things. It can hold on to many compounds for a long time. There was a study in the Midwest where researchers went into farmers’ homes 50 years after DDT was banned and found it in 50 percent of homes. Ventilation has little effect on heavy, sticky molecules like DDT. Other heavy compounds include other pesticides, phthalates and brominated flame retardants. They get stuck on surfaces. So indoor air is acted upon by a large reservoir of surface compounds that won’t come off substantially due to ventilation alone.

But there are lighter compounds that, if you open up the house and let it breathe, you can flush a lot of those out, or keep their concentrations down.

What are some common sources in our homes?

Some pollution is from continuous sources, such as emissions from building materials or things like plug-in air fresheners. Or they’re tied to activities such as cooking, cleaning, using space heaters, using humidifiers, burning candles. These sometimes add to indoor air pollution greatly.

Acrolein — a compound that is due to burning fat, such as meat fats — is emerging as a compound of strong interest that should be investigated more.

We’re also doing work now on a certain type of humidifier — ultrasonic, which uses vibration to put moisture in the air. It’s recommended that you use distilled water for these. But probably most people use tap water, and tap water has all sorts of solids in it. If you aerosolize that water, you’re aerosolizing those solids, as well. We’ve done preliminary tests and found that you can get concentrations of particles in a room that are the equivalent of a very polluted city, such as New Delhi or Beijing.

You mentioned plug-in air fresheners as a problem. What’s wrong with them?

There’s a huge market in the U.S. for scented products. Marketing has convinced us that clean is something you smell, that clean has a pleasing scent. But it means you’re actually dirtying the air by adding compounds to it. One is d-limonene, which is the smell of oranges. The smell of oranges is nice. The smell of lavender is nice. The problem is that when we emit these things, the molecules react with ozone that’s already in the air and ultimately end up forming particles and other pollutants in the air.

Plug-in air fresheners are masking agents. In my opinion, clean air is the absence of any odor.

I’ve read that houseplants can help clean the air. Is this true?

A lot of people like the idea of plants cleaning the air. Isee it all over the internet. Those claims are inspired by a study from the 1980s that has since not been supported by other, more recent work.

Drexel has a biowall — a wall of plants — that actively pulls air through the root system of the plants. It’s moving air at a controlled rate through the plant roots, using a fan. It turns out the plants will remove some small fraction of volatile organic compounds in the air using their leaves, but 90 percent of any VOC removal happens in the soil and roots, because of the bacteria that live on them.

That is very different from my getting a pothos and putting it in my office and thinking it will clean my air. Newer modeling studies show that to have an impact, you would have to have so many plants it might be difficult to move around in the space. sandybauers10@gmail.com