Assessing Outdoor Air Near Schools
VOC Resampling at Selected Schools: Additional information for those who want to know more
Q: How did the VOC samples become contaminated?
A: A leak in a fiberglass box housing a timer for the monitor allowed air from inside the box – not just air from the outside -- to get into the canister containing the air quality sample. The air inside the box contained some chemicals, which were emitted by the box and its contents. When the timer box leaked, those chemicals contaminated the samples. As a result, when the samples were analyzed, it appeared that there were pollutants in the air around some schools that 1) may not be there at all, or 2) may actually be in the air, but at smaller concentrations.
The VOC monitors were equipped with a fiberglass box housing an electronic timer. That box leaked where it attaches to the monitoring equipment, allowing air from inside the box – not just air from the outside -- to get into the canister containing the air quality sample. So when the samples were analyzed, it appeared that there were chemicals in the air around some schools that 1) may not be there at all, or 2) may actually be in the air, but at smaller concentrations. When the timer box leaked, all or some of those pollutants were coming from inside the timer box.
Normally, the VOC sampling systems work like this: Air is drawn into a specially treated stainless steel canister. A timer (housed in the box) opens a valve that allows outdoor air to flow through a very narrow tube down into the canister. After 24 hours, the timer closes, stopping the outdoor air from entering into the canister. Once the canister is retrieved from the field, it is sent to a laboratory and is analyzed by instruments that identify each pollutant and measure how much is present.
How the contamination occurred: The electronic timer is housed in a fiberglass box. That box is attached to the sampling system by fittings. If the fittings are tightened properly, then only outside air enters the canister. But if the fittings are not tightened properly, they can create a leak that allows air from inside the box to get into the canister that contains the sample of air. That’s a problem, because the air inside the box can contain gases that are emitted from the fiberglass housing and components inside the housing. This is called “off-gassing.” (It is much like the new-carpet-smell, which comes from off-gassing of chemicals that go into making carpet).
The solution: States and local agencies are addressing the timer leak by either 1) removing the timer from the monitoring equipment and manually opening and closing the valve on the canister that collects the air or 2) having the timer retrofitted with a stabilizer that prevents the fittings from loosening.
Q: How did you determine which schools would re-monitor?
A: When EPA’s lab determined the timers on some monitors were leaking, their analyses of clean air showed elevated levels of certain chemicals – especially acrylonitrile and dichloromethane.
We used this information as we examined the monitoring results for all schools that had measured VOCs. As we looked at the data, we saw a distinct “signal” that indicates a sample is likely contaminated. This signal is elevated levels of two chemical compounds – acrylonitrile and dichloromethane -- that are not typically seen at in the outdoor air.
Then we compared the results to data from our National Air Toxics Trends Stations network for recent years (2004 to 2008) to further help us determine whether results were in the range of levels likely to be found in the outdoor air.
We also checked with the people operating the monitors to find out if they had noticed problems with the timer device; and we checked with our laboratory to determine whether there were any issues that might call the data into question for other reasons.
If the data show signs that the timer was leaking, we invalidated samples collected with that monitor -- from the beginning of the monitoring until the timer was either 1) removed; or 2) sent to EPA’s lab to have a stabilizer installed that prevents the leak from occurring.
As a result, additional VOC samples will be collected at 24 schools. EPA has determined that samples from an additional 17 schools monitoring VOCs were likely not contaminated; however, monitoring will be extended at one of those schools at the request of the local air quality agency.