Indoor Air Monitoring
When there is a potential for vapor intrusion, indoor air sampling is considered by many stakeholders to be the most definitive method of determining if there is actually a problem. If indoor air testing is not possible, the alternative is to take a subsurface soil gas sample below the home or commercial establishment (see description of sub-surface soil gas monitoring) and model potential indoor exposures. Ideally, indoor air testing, soil gas sampling, and ambient (outdoor) air testing are conducted simultaneously to determine if elevated indoor air concentrations are from the subsurface, indoor sources, or the background outdoor air.
There are several technologies for monitoring indoor air, including:
á Obtaining Grab Samples
á Real-Time and Near-Real-Time Sampling
á Passive Sampling
The most common methods for monitoring indoor air are through grab samples. Summaª Canisters are used most to collect samples, followed by Tedlar¨ Bags. Other methods, such as carbon absorption tubes, are gaining in popularity.
Summa canisters are small airtight metal containers. The sampling canister is sent to the field under vacuum and certified leak-free. The canister fills with air at a fixed flow rate over a preset period of time with use of a flow controller. The appropriate preset period should be determined on a case-by-case basis. For example, in residential settings, the period is typically 24 hours. In commercial and industrial settings, samples are normally collected over 8 hours to correspond to an average workday. After the canister is filled, it is sent to an analytical laboratory under the appropriate chain-of-custody protocol and analyzed. Very low detection limits, in the subparts per billion volume (ppbv) range are possible.
Tedlar bags are a suitable alternative to Summa canisters for volatile organic compounds (VOCs) when the holding time is less than two weeks and concentrations are in the parts-per-million (by volume) range. Tedlar bags are made from polyvinylfluoride film. They require the use of an air pump, so proper pump calibration is critical. They are lighter and less expensive than canisters. However, some regulatory agencies may not accept results.
Real-time and Near-Real-Time Sampling
Real-time and near real-time analyzers can be used to collect multiple samples that can locate problem structures, vapor migration routes into structures, and volatile sources inside the structures. Continuous analyzers that collect data automatically over a period of time can sort out background scatter and determine temporal variations both indoors and below ground (soil gas).
A variety of real-time analyzers exist, including handheld logging instruments (see Photo Ionization Detector), automated Gas Chromatographs (GC) and portable Mass Spectrometers (see Gas Chromatography/Mass Spectrometry or GC/MS). Several mobile laboratories offer analysis by GC/MS with selective ion monitoring to obtain accurate results measured in the subparts per billion by volume range.
U.S. EPAÕs Trace Atmospheric Gas Analyzer (TAGA) is a self-contained mobile laboratory (van) capable of real-time sampling and analysis in the low ppbv level. This unit is outfitted with a long hose and vacuum pump that allows the operators to survey a building in real time and identifies interior sources of chemicals. The unit is equipped with several gas chromatographs to aid in identification and confirmation of analysis, and it can be equipped with other instruments such as portable GC/MS for organic compounds or X- Ray Fluorescence sensors for inorganics (metals). The TAGA unit can also carry equipment (e.g., Summa canisters, Tedlar bags) for quick on-board laboratory analysis to supplement the results of other samples. However, the cost ($10,000 per day) and limited availability of the TAGA hinder the use of this system.
Passive samplers, less commonly used for vapor intrusion assessments, are similar to those described in Passive Soil Gas Screening. Samplers use an adsorbent to capture VOCs over a set period of time. The samplers are sent to a laboratory for analysis. With some samplers, the rate of sorption is controlled and thus reasonably known, and the average concentration can be estimated. In the qualitative samplers, the rate of sorption is not controlled, so the mass captured can only be related to concentration using other relationships (such as air exchange rate, the geometry of the room, etc.). Passive sampling has several potential advantages over other methods, including lower cost, small size, and less disruption to building occupants. Disadvantages include weaknesses in detecting semi-volatile organic compounds (SVOCs) and less analytical precision. Often passive samplers are used to screen a building, and if a problem is indicated, other methods are used.
Limitations and Concerns
It is essential to use methods capable of measuring levels of contamination at health-based thresholds. For some common toxic vapors, such as trichloroethylene and perchloroethylene (also known as tetrachloroethylene), those are in the parts per billion by volume range. Thus, hand held PIDs are generally not suitable.
Results are usually reported in parts per billion by volume or micrograms per cubic meter (µg/m3). The conversion factor between the two units depends upon the molecular weight of the compound in question.
One-time sampling results may not be representative of long-term average concentrations. As an added precaution, indoor air samples may be collected using two scenarios: first with the building under an induced positive pressure and second with the building under negative pressure relative to the sub-surface. Under consistent positive building pressure, vapor intrusion is likely to be insignificant, and the indoor air concentrations are influenced only by indoor and outdoor vapor sources. Under an induced negative pressure, air will flow into the building from all discontinuities in the building envelope, including the subsurface, thus enabling one to calculate the relative contribution from the subsurface by the difference between the indoor air concentrations measured under positive and negative building pressure. In the eastern U.S. or the Midwest, this may be similar to testing in the summer and winter.
In some instances the goal of the monitoring program may be to indicate the worst possible exposure, while in other instances the goal is to indicate exposure under normal conditions. Goals should be clearly defined because they could affect the timing and frequency of monitoring. For example, in colder climates, indoor air vapor concentrations may be increased due to high negative pressure from indoor heating (furnaces, etc.). There is some controversy whether the best approach to sampling indoors during is to leave heating and ventilation systems on or off. With the former, one can achieve data that reflects what occupants are exposed to. In the latter instance, data may reflect the worst exposure. In some instances, data has been collected with the heating and ventilation both on and off.
Indoor air and ambient air concentrations tend to exhibit considerable degrees of variability over time and under different climatic conditions. For example, in northern climates frozen ground may limit vertical flow of vapors, thus increasing the lateral flow under buildings. Increased moisture due to rainfall and snowmelt may also serve to drive vapors into the relatively dry soils under a building.
Where it is possible, sample locations should be selected in breathing zones and around potential preferential pathways. Concentrations within a building are typically higher in the lower level. Therefore, indoor air samples should be collected in the basement, if present, or on the first floor. If there are pathways within a building that may induce vapors to flow to upper floors (e.g., elevator shafts), these should be sampled.
There is a wide variation in building sizes, features, and age that will influence the location of samples. For instance, older homes are less likely to have vapor barriers incorporated into their foundation/slab construction, as well as more likely to develop cracks that allow for infiltration of vapors. Earthen floors and those of limestone provide increased opportunity for vapor intrusion. Chronic wet basements are also a potential source of vapor intrusion as contaminants in dissolved groundwater can off-gas into the indoor air.
It is recommended that an upwind (from the building being sampled) outdoor ambient air sample be taken during every indoor air study to be used in the final evaluation of potential sources of chemicals in the indoor environment. Generally, if background air is higher than risk-based values for indoor air, there is little point of mitigating indoor vapor intrusion. A finding such as this often suggests that efforts should be accelerated to remediate the subsurface source of vapors.
For there to be an accurate sample, products or operations that produce vapors that are being tested for must be removed from the building. These sources include cleaning products, hobby supplies, paints, recently dry-cleaned clothes, and a host of other common items, including gasoline stored in garages or adjoining structures. Even closed containers may release low, but measurable levels of target contaminants.
For passive air sampling, exposure periods longer than one week may be required. Longer sampling periods may be difficult because of the expense, the lack of control over the situation (i.e., tampering or the addition of variables or new sources by unsupervised building occupants).
Sampling equipment should be placed to minimize potential contamination from extraneous sources such as gasoline stations, automobiles, and other gasoline-powered engines, outdoor oil storage tanks, industrial facilities, etc.
EPA suggests that if a building is located within 100 feet of a source (either vertically or laterally), vapor intrusion cannot be ruled out. In sites that overlie fractured bedrock or coarse-grained soils, that range may be greater.
Petroleum hydrocarbons (such as benzene) biodegrade well in unsaturated soils. This mechanism should be considered in the screening of buildings to be sampled.
Unnecessary ventilation should be avoided 24 hours prior to sampling.
The methods described are used to collect indoor and outdoor air samples.
Technology Development Status
The sampling methods described are all commercially available.
See http://www.mass.gov/dep/cleanup/laws/02-430.pdf for an excellent description of various air-sampling protocols.
Other Resources and Demonstrations
See "A Stakeholder's Guide to Vapor Intrusion," by Lenny Siegel (see http://www.cpeo.org/pubs/SGVI.pdf).
See http://www.epa.gov/ttn/amtic/files/ambient/airtox/to-15r.pdf for a description determination of ambient air concentrations, primarily using canisters.
See http://chromatographyonline.findanalytichem.com/lcgc/article/articleDetail.jsp?id=409516 for a detailed description of Electron-Capture Detectors (ECD). Also see http://www.youtube.com/watch?v=d_6ON7TitTM for a short animated explanation.
See http://www.itrcweb.org/Documents/VI-1.pdf and http://www.epa.gov/nrmrl/pubs/600r08115/600r08115.pdf for a full discussion of vapor intrusion technologies and regulatory issues.