No Images? Click here Vapour IntrusionIn this ongoing series of bulletins Mort Schmidt of Cox-Colvin & Associates (US) explores the complex world of vapour intrusion, from the fundamental science through to legislative framework. In this first bulletin the contribution of ambient air to background sources of contamination are explored. This article cites a number of US examples, with the Australian perspective added where possible. Contamination from ambient airAir is composed almost entirely of nitrogen, oxygen, and argon, with variable amounts of water vapor. And because the remaining components make up less than 0.1% of the atmosphere, minor changes in conditions can cause dramatic shifts in their relative concentrations. That’s one reason why vapor intrusion behaves so unpredictably. Volatile Organic Compounds (VOCs) are nearly always present in indoor air and come from ambient air, indoor sources known as “background”, and in some cases, soil gas, which we call “vapor intrusion”. Therefore, to eliminate one of the variables, indoor air sampling should be accompanied by ambient air sampling to see what’s in the air as it enters the building. Benzene is one of the top offenders in ambient (and indoor) air, because it’s both common and relatively highly toxic. Benzene is ubiquitous due to its presence in petrol (in which it can be at concentrations reaching 4-5% v/v), and unfortunately, often present in ambient air at significant levels. According to Ohio EPA’s All Ohio Air Toxics Report, in Jefferson County, Ohio, the average ambient benzene concentration from 2004 to 2009, was 7.1 micrograms/cubic meter (ug/m3), which exceeds EPA’s Vapor Intrusion Screening Level (VISL) Calculator’s indoor air target level of 3.6 ug/m3 in a residential setting. The oft-discussed trichloroethene (TCE) level in ambient air was 0.39 ug/m3, compared to the residential VISL of 2.1 ug/m3. A range of Australia studies have quantified the level of contamination of ambient air, both indoors and outdoors. The NEPM (Air Toxics) provides guidance on levels which warrant further investigation (monitoring investigation levels). Benzene, as an example, is 0.003ppm or 9.57 ug/m3. A survey of Melbourne houses found concentrations varied from 0.9 (outside) to 1.5ug/m3 (inside) (CSIRO 2010). Similarly, a survey of ambient air in the Sydney CBD found an average of 7ug/m3 (DEC 2004). The handling of ambient air results is interpretive. You can’t simply subtract the ambient vapor concentrations from indoor air, but with other information, you can make the case that vapor intrusion contributes only part, or even none of the vapors. The radar plot from a recent vapor intrusion investigation conducted by Cox-Colvin shows an obvious similarity in concentrations between indoor air (blue) and ambient air (purple) for the BTEX compounds (benzene, toluene, ethylbenzene, and xylenes) and 1,2,4-trimethylbenzene (1,2,4-TMBZ), all of which are petrol constituents. Even though TMBZ was also present in soil gas (red), the consistency between ambient and indoor concentrations for BTEX and TMBZ, together with other data, makes a compelling argument that most of the indoor TMBZ came from ambient air.
The contribution of ambient air to indoor contamination must be taken into account. The combined hazard index (HI) for non-carcinogenic VOCs in ambient air in Franklin Ohio, according to Ohio EPA’s report, is 0.49 compared to a limit of 1. The combined risk for carcinogenic VOCs in ambient air is 7.3E-5 (one in 14,000 excess cancers), which exceeds the limit of 10-5 (one in 100,000 excess cancers). This is why we normally collect ambient air concurrently with indoor air, and we collect it upwind of the building so that it reflects air entering the building. It also goes to show that vapor intrusion is only one source of contamination in indoor air. In next month’s bulletin, we’ll talk about another source – background. About Mort Schmidt Mort Schmidt is a Senior Scientist with Cox-Colvin & Associates, Inc. He received his BS and MS degrees in Geology and Mineralogy from The Ohio State University, and has been a Cox Colvin & Associates employee since 1997. His areas of expertise include vapor intrusion and contaminant investigation and analysis, and he currently serves as Cox Colvin's Practice Leader - Vapor Intrusion Services. Mort is a Certified Professional Geologist with AIPG and is a registered Geologist in Indiana. Vapour Pin by Cox-Colvin
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