The word volatile has many meanings.
It can, for example, convey the violently responsive and on-edge –as in, a volatile person, or a volatile situation to avoid. Volatile can describe an object that is elusive, or one that can’t be held down. It can mean a thing that flies, as in the Spanish volador. Or it can mean, in the most scientific and precise terms, a chemical substance that is poised to evaporate at normal temperatures.
The word volatile in the phrase volatile organic compounds (or, VOCs) signifies a little of everything described above. VOCs are chemicals that evaporate so easily at room temperature –bottled up by molecular pressure, atoms on-edge– that we almost never see them as liquids. Instead, they exist freely as gases in the air. You cannot see them, but volatile compounds are already in flight, transient, ready to react with whatever they bump into. This is part of their problem, but we will get to that later.
Organic is another word like volatile that can mean several things, but here, it means something very specific: a chemical that contains the element carbon. And compound, a joining of several molecules into one.
Intercepting chemical messages
I learned about VOCs as I learn about most things –through a plant. This time it was sagebrush, a woody shrub skirting the grounds of the Sierra Nevada range. An ecologist from UC Davis had published a paper about a curious interaction between a sage and tobacco plant neighboring each other on the same hill. When he clipped the leaves of the sage to mimic grasshopper damage, he found that the nearby tobacco plant seemed to sense it as well, turning up its own grasshopper defense responses.
At first, this did not make sense –that is, until he realized that the injured sage was releasing VOCs into the air around it, and that the neighboring tobacco was “eavesdropping” to pick up the signal of danger. The idea of an invisible layer of chemical information floating around our heads is a fascinating one indeed. In the case of plant defense, VOCs act as coded messages –hints of danger that may be intercepted, interpreted, and finally acted upon by other plants, insects, birds, and microbes hoping to mitigate the situation.
We do not make VOCs from our bodies as plants do from theirs, but we do produce them. We create VOCs indirectly via the tools, processes, and industries that we have developed over the course of our human history. In terms of environmental health, manmade VOCs are unambiguous pollutants. They arise in various forms from traffic, industry, consumer products, smoking, and the burning of fossil fuels. Though the mechanism of their production is very different, manmade VOCs are no less signals of environmental danger than are their plant counterparts.
On warm days, sunlight allows VOCs to combine with another airborne pollutant by the name of nitrogen oxide. Together, the two chemicals form ground-level ozone. This is not the so-called “good ozone” that forms a protective layer in our atmosphere, blocking out the sun’s harmful rays. Ground-level ozone is what you might call “bad ozone,” the main ingredient in smog. It can cause respiratory distress and other chronic health problems, particularly within vulnerable populations like older adults, young children, and those with asthma.
Of course, VOCs are just one piece of a much larger public health puzzle. Other pollutants like dust and particulates also clog up our air and put people at risk. But VOCs are uniquely helpful as markers because, as specific and identifiable chemicals, they give us information about their possible sources.
Mapping neighborhood air quality
In 2013, the City of Minneapolis’ Environmental Health division pitched the idea of examining VOC levels in all of the metro area’s neighborhoods. They envisioned a multi-year project with hundreds of data collection points for a truly comprehensive local picture of air quality.
The details seemed straightforward: they would collect air samples in stainless steel devices called Summa canisters, which bear some resemblance to the kettle bells you might use at the gym, and have the samples analyzed for VOC levels at a lab. But for the plan to work, every canister would need to be opened and closed at roughly the same time, and there were only 5 employees on hand to do it.
Rather than allow the project to fizzle out before it could even start, the team enlisted the help of 130 volunteers and 10 local businesses –autobody shops, drycleaners, industrial printers, and more. The volunteers were trained in data collection, given a canister, and sent off to work. These volunteers collected a full 900 samples over the course of 2 years.
The city was then divided into a grid of “zones,” with each zone receiving at least two data points of representation. Some communities pitched in to sponsor additional collection canisters for peace of mind–for example, the SE Como neighborhood, whose residents some years ago were informed that their homes sat upon a heavily contaminated “superfund” site after General Mills had improperly disposed of a chemical degreaser in the area decades before.
Overall, the results of the study were reassuring. Minneapolis, for the most part, does not suffer high VOC pollution. The air around here is good, and most readings fell well below the very conservative health benchmark levels used for measurement.
Still, there were elevated single readings of some VOC chemicals at some measurement sites. These spiked measurements do not necessarily represent a widespread problem, but they may point to localized sources of contamination that we could begin to address. To understand the sources, though, we must first understand the individual VOCs that appeared at higher levels:
Benzene is colorless and highly flammable. It floats in water and sinks in air. The British chemist Michael Faraday first discovered it in a flame –the oily residue left over from a street lamp’s gas. In fact, benzene is found wherever there is something burning. Picture it in a smoldering volcano, a forest fire, wicking out from gasoline vapors.
Today, half of all benzene exposure comes from cigarette smoke and tobacco. Another 20% comes from your car’s tailpipe.
The six carbon molecules of benzene situate themselves in a neatly spun ring. The scientist who proposed its cyclical shape saw it all at once in a dream after many weeks of struggling, a snake swallowing its own tail. We now recognize benzene’s simple ring as a modern building block with endless potential. Where once it was produced primarily as a byproduct in the steel mills, today it is made quite intentionally for its use in the production of plastics, resins, nylons, rubbers, dyes, detergents, drugs, and fibers.
It seems that benzene has been used at one point for nearly everything. At the same moment in our ever-curious human history, it was being used as an industrial solvent, a coffee decaffeinator, and an after-shave lotion. Some say that it has a sweet and pleasant smell. A 1969 report describes its odor as “solvent.” Elsewhere, a poet describes it as “lilac.”
Ominously situated in the New World Encyclopedia between the entries for napalm and Napoleon Bonaparte, napthalene is a doubled benzene –two rings. Like its shape, it seems to have two characters: help and harm, light and dark. For example, two places that you can find napthalene are in coal tar and in magnolia petals.
It is a bright white salt by itself, so that it might be startling to discover that it originates in the thick, sticky blackness of fossil fuels. Napthalene has the smell of mothballs, one of its most common uses. It is also a major component in the production of PVC piping. Environmentally, most napthalene is the result of burning fuel, wood, and tobacco.
Trichloroethylene, sometimes called TCE, has had many lives. It was first produced in the 1920s as a means of extracting vegetable oils from plants. Later, it was marketed as an anesthetic to replace chloroform and ether, until it too was found to be wildly unsafe. It was used for a time in drycleaning, before being replaced by its close cousin tetrachloroethylene with its extra chlorine atom. Otherwise it has been paint stripper, spot remover, rug cleaner, typewriter corrections fluid, film conditioner, rocket engine cleaner, and, most traditionally, metal degreaser.
PERC is a colorless, but not odorless, dry-cleaning fluid. Its odor is detectable by most people at 1 part per million (ppm).
1 part per million is 4 tablespoons of water in a swimming pool.
1 part per million is the thickness of one human hair beside the height of the Statue of Liberty.
1 part per million is the weight of a single jelly bean beside the weight of a brown bear, or two grand pianos.
1 part per million is 31 seconds out of a year.
Formaldehyde is a gas at room temperature. When mixed into solution, it works as an embalming fluid. Like benzene, it is a natural byproduct of combustion. Otherwise, it is used in the surface coatings of particleboard, plywood, fiberglass, laminates, and some synthetic fabrics. Formaldehyde is consistently measured above health benchmark levels across the state of Minnesota, particular in areas with heavy traffic.
Making sense of the results
Right now, you can view interactive maps online to see the areas in which each of these VOC measurements were higher than health benchmark levels, and by how much. The size of each dot corresponds to the level of detection, like a stone tossed into a pond will produce ripples based on its heft.
The next step –due early spring– will be to produce a mapped model that extrapolates air quality across the region and that takes land use into account. This will allow us to pair contamination patterns with patterns of land use (e.g. traffic, industry, etc). Community-specific public health data is vital because it will allow the City to examine (and hopefully mitigate) environmental inequities where we live and work.
Air pollution has dropped by nearly half in the past 30 years, but there is still work to do. Airborne pollution is responsible for about 2,000 deaths annually in the Twin Cities, and many hundreds more emergency room visits. Likewise, because people of color are disproportionately affected by diseases like asthma, it is as much an issue of social justice as it is a means of keeping track of our environment. Not every Minnesotan breathes the same air, and it is important that we keep proper track of disparities and work towards correcting them.
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