Environmental use of diffusive samplers: evaluation of reliable diffusive uptake rates for benzene, toluene and xylene

Abstract

The measurement of workplace air quality is a key part of an exposure assessment in relation to legislative requirements such as, for Europe, the Chemical Agents Directive (98/24/EC).¹ For volatile organic compounds (VOCs), diffusive sampling methods based on a thermally desorbable tube design² have been used by the Health and Safety Laboratory (HSL) for many years as the primary measurement technique and the procedure is well established in Health and Safety Executive (HSE)³ and international⁴ standards. The assessment of outdoor air quality is just as important. Research at HSL,⁵ and corroborated elsewhere,^6-14 has demonstrated that tube-type thermally desorbable diffusive samplers, such as the Perkin-Elmer sampler, can be used effectively for monitoring VOCs, particularly benzene, toluene and xylene, in outdoor air. These pollutants arise primarily from vehicle exhaust, and so are of greater concern in the urban environment. Of the three, benzene is the most emotive, because of its carcinogenic potential. In most European states, these three pollutants are measured by fixed monitoring stations. However, diffusive sampling can be implemented at a fraction of the cost of fixed monitoring stations, and can cover a larger number of typical sites. Moreover, they give long-term averages, which are more consistent with the proposed European limit value which is likely to be set on a yearly basis. Diffusive sampling can also be used for monitoring indoor air quality,¹⁵ where VOCs are implicated, along with formaldehyde and many other factors, in ‘sick building syndrome’. There is no consensus, as yet, as to whether ‘total VOCs’ (however they are defined) or specific individual VOCs are to blame, nor have any firm limit values (LVs) been set. However, measurements in identified sick buildings are about two orders of magnitude below workplace LVs. Usually, such concentrations are measured in relation to outside air, which is considered relatively innocuous. The concentration range of interest for indoor air is, therefore, well within the capability of diffusive samplers, which, for ambient air, can measure two further orders of magnitude lower. This is done by the simple expedient of exposing the samplers for a longer time: typically, samplers are exposed for a few hours for workplace measurements, a few days for indoor air and a few weeks for outdoor air. It is well known, however, that the diffusive uptake rate of samplers can be dependent both on the time of exposure and the concentration to be measured.¹⁶ This is particularly the case for tube-type samplers with sorbents designed for thermal desorption. Uptake rates for workplace applications (i.e., for exposures in the 1-100 ppm range for 1-8 h) have been published³ for a wide range of VOCs. A shorter list for environmental applications (i.e., for exposures in the 1-10 ppb range for 1-4 weeks) has also been published,¹⁷ but from a limited number of sources. Tables 1-3 document known data for three commonly measured environmental pollutants: benzene, toluene and xylene (not distinguishing between isomers). These data demonstrate good agreement between the independent estimates of uptake rate, bearing in mind that the practical uptake rates will be influenced by the experimental conditions of temperature and concentration. The magnitude of this error can be estimated from the mean of the standard deviations, which is about 10%. In most cases, ambient uptake rates have been determined at concentrations typical of urban background levels, i.e., a few μg m^–3 of benzene, toluene or xylene. Rates have been determined either (NMI 1 and BRE) in the laboratory with test atmospheres themselves checked with an independent method (usually a pumped sorbent tube) or (other data) directly against one or more samplers of an independent method under field conditions. The BRE data give uptake rates for indoor air, where the total VOC concentration level of the generated test atmosphere was 2 mg m^–3, which is typical of the mean 4 week concentrations found in urban homes after recent painting and decorating activities.

References

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