What is Soil Gas, and Should I Care? Only if You Breathe Indoor Air

By Louis S. Moore

As we enter the summer months, we can expect to again hear of air pollution alerts. This is due to ground level or "bad" ozone that is created by chemical reactions between nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. Emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline and heating oil vapors, dry cleaning fluid and other industrial solvents are some of the major sources of NOx and VOCs. Exposure to VOCs can have adverse health effects depending on the length and type of exposure and the toxicity of the chemical.

In times of these alerts, we are advised to reduce outdoor activities and stay indoors. Yet indoor air pollution can be much worse than outdoor air pollution. The possible sources of such pollution are numerous: radon, mold and tobacco smoke, recently dry cleaned clothes, new carpeting or furniture, improperly vented space heaters or woodstoves, and household items such as paint, varnish or cleaning products.

Another, often overlooked source is from outside the home or workplace -- vapor intrusion from VOCs in oil and many hazardous materials that contaminate soil or groundwater. These contaminants can evaporate or volatilize into air spaces between soil particles. In undeveloped areas, the vapors disperse into the outdoor air. But in developed areas they may seep into buildings through concrete floors. Warm air can draw vapors up through foundation cracks and utility conduits where they contaminate indoor air.

There is increasing focus among environmental regulators and practitioners on vapor intrusion at or in the vicinity of sites where VOCs such as petroleum or solvents are present. The potential for vapor intrusion should be assessed in light of relevant site uses. Contamination may not pose a significant risk to health or safety at a property in its undeveloped state or in use as a paved parking lot. However, that is no assurance that the same is true should the property be developed for a residence or commercial building. In addition to soil and groundwater sampling, testing of soil gas -- the gaseous elements and compounds between soil particles – is a method to determine if vapors present under a building, or a future building, could cause a problem. Indoor air testing is also often done, but because of other possible sources of indoor air contamination (some mentioned above) or conditions (open windows or ventilation systems) it may not be reliable in identifying subsurface sources.

Should vapor intrusion be a problem, sealing foundation cracks or modifying heating and ventilation systems may resolve it. Additionally, installing a sub-slab depressurization system can draw subsurface vapors from below a foundation and vent them to outside air.

If you have a question about an indoor air pollution issue or environmental law in general, please contact Attorney Louis Moore at (413) 732-6400.

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