Vapour Intrusion

Although air quality issues typically result from direct emissions from exhaust stacks or chemicals used in industrial processes, vapour intrusion is caused by contaminants in soil or ground water volatilizing (evaporating) and moving indoors through the foundation or floor slab. Vapour intrusion, particularly of volatile organic compounds (VOCs), is increasingly recognized as a potential source of indoor air contamination. Since strict regulations on dumping, spill handling and usage of chemicals were not in place until the mid-80s, many industrial sites and landfills near residential areas were inadvertently contaminated before these regulations came into effect. Contaminants in soil or ground water persist for many years and are slow moving, thus, problems may only appear after many years and may lead to serious indoor air quality issues, potentially exposing occupants and, in the long term, their health if concentrations are above acceptable limits.

Many areas with vapour intrusion potential are identified during Phase I and Phase II assessments mandated when commercial or industrial properties change hands. This increased surveillance, coupled with advances in both measurement technology and toxicological risk assessment about low-level, chronic exposures, has identified many sites with potential vapour intrusion problems.

Although assessment of ground water and soil vapour contamination normally leads to identifying areas potentially affected by vapour intrusion, ultimately the exposure of occupants is of most concern and the indoor air needs to be assessed to ensure that exposures are within acceptable limits requiring?direct measurement of indoor air.

Traditionally, three types of methods are used to assess indoor air levels of VOCs. No method is perfect or suitable for all compounds or in all situations, so it is important to select the one that satisfies the requirements of the assessment for the target compounds. Moreover, indoor levels fluctuate due to many factors including diurnal variations, building activities and occupancy. Also, as weather systems move over contaminated areas, the passage of successive low and high-pressure systems results in a pumping effect that causes indoor levels to fluctuate. To determine the concentrations of the target compounds, the collection method should have sufficient sensitivity, accuracy and precision over the required assessment period.

The Typlical Methods Available for Indoor (and Outdoor) Measurements are as Follows

Canister sampling
Canister sampling entails using a stainless steel canister with a passivated inner surface and inert inlet and outlet valves. The canister is cleaned, tested for purity and evacuated, fitted with a calibrated orifice, and activated at the measuring point. Typically, after 24 hours the canister valves are sealed, and the air is analyzed with GC/MS and cryogenic focusing, for example, to determine the concentrations of target VOCs.

Sorbent tubes
Sorbent tubes are deployed and contaminants collected with a calibrated, battery-operated sampling pump. Sorbents are selected to collect and ultimately recover the target contaminants with thermal or solvent desorption. Then, the extract or eluent gas stream is processed with a GC/MS, for example, to determine the concentration of target VOCs.

Passive samplers
Much like the sorbent tubes above, passive samplers with specific sorbents are selected to collect and ultimately release target contaminants. However, rather than using a sampling pump, passive samplers collect contaminants by passive air diffusion on the sorbent at known rates. Samples are processed in a similar manner to that for regular sorbent tubes.

Each of the methods has advantages and disadvantages. Canisters and solid sorbent tubes generally provide the best sensitivity. So, if sensitivity is a major consideration and can not be adequately addressed otherwise, then these should be the methods of choice. In contaminated indoor environments this is generally not an issue, however. Canister and solid sorbent methods can be noisy and obtrusive indoors and may not be suitable for measuring the broad range of concentrations that occur. Moreover, they can be very expensive to use. The cost issue can limit their usefulness in large-scale surveys sometimes necessary to delineate the extent of sub-surface contamination. The analysis methods can also be complex and lengthy, resulting in long reporting intervals. This can make residents anxious as they await results about their living or work space.

Passive sampling alleviates many of these concerns and deficiencies. Passive samplers are small and unobtrusive. In addition, since they require no pump, they make no noise. They are easily deployed and retrieved, and in general meet the requirements for accuracy, precision and sensitivity for most target compounds. Moreover, handling and processing is simplified so costs are reduced significantly and response times are quicker.

Airzone has developed specific expertise with passive sampling devices (PSDs) as a result of over 30 years of development work in our test atmosphere generation systems. We have used PSDs in large scale surveys, in diagnostic studies, for evaluation of personal exposures, in traffic studies, etc., in cooperative projects with governments and universities. With this extensive experience and through careful development of handling and analysis procedures, we have developed protocols with measurement attributes (precision, accuracy and detection limits) that meet Ministry of Environment guidelines. Moreover, PSDs allow variation in sampling intervals, allowing not just daily sampling but extended sampling intervals (1-week, 1-month) more suitable for toxicological evaluations of exposures. By using this method, we are able to deploy fewer specialists in the field and improve the speed for reporting results.