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The article by Harrison and colleagues1 reports on a relation between personal and static microenvironment air sampling for carbon monoxide and nitrogen dioxide and for PM10 which include the addition of a “personal cloud increment”. Static sampling is also commonly referred to as area or stationary sampling.2,3 These relations are important because static sampling is more easily achieved than personal measurements and is generally less costly. To achieve a relation for personal and static sampling they must be collected from the same pollutant population.4–7 Thus, in establishing a microenvironment or personal cloud increment, there must be a relation within the sampling location for the pollutant.
Previous occupational studies have noted no relation2,4,8–11 and a relation12,13 between personal and static sample measurements. As mentioned by Harrison et al, personal samples are generally higher in concentration than static samples because of people being closer to the source and spending more time within the source location, or in the emission pathway.4,14 When static samplers are placed at the source location or emission pathway they are similar to the values reported for personal samples,2,3 and in some incidents may exhibit a higher concentration.4,13,15
The relation reported by Harrison et al, for CO and NO2 is likely a result of these pollutants being a gas, their ability to diffuse, low reactivity, and similarity in concentration between indoor and outdoor environments. A personal cloud factor must be incorporated into the PM10 measurement because of greater variability of concentration from location to location.16 A microenvironment represents a similar location and the personal cloud is a correction factor extrapolating for the static exposure to personal measurements. It must be noted that this adds a degree of uncertainty in extrapolating exposure from one sampling method to the other. Even though static samples may be reported as similar, they will ultimately exhibit a lower concentration than personal measurements.
Harrison et al provided summation of their data in the form of arithmetic mean (AM) and standard deviation. When data from tables 2 and 3 in their paper were evaluated for form of distribution, using the Shapiro-Wilk test,17 most exhibited a non-normal distribution (see table 1). However, due to the small number of samples in the data of Harrison et al, the actual form of distribution cannot be determined. It is suggested2,18 that the logarithmic form best represents airborne pollutants, including the data of Harrison et al. When providing pollutant data, it has been suggested to include summary statistics representative of its form of distribution.2 Data should be shown as AM, standard deviation, range, geometric mean, and geometric standard deviation (GSD).2,12 It has been suggested19,20 that health effects from exposure are more closely related to AM values, especially for those that are chronic in nature, making AM an important summary value to report. Reporting all summary statistics will allow future investigators to select summary data most relevant to their purpose.
Since many environmental pollutants are distributed throughout a location, such as the home, modelling will prove useful in establishing a relation between personal and static samples. However, this relation may not only depend on sampling locations and emission pathways, but on the actual pollutant as well.6
Variability among samples must also be considered when predicting exposure levels. Most sample populations exhibit a GSD (day to day variability) of 2.0 to 3.0.2 The probability of samples with this variability being “related” is about 17–28%.21 The GSD for the data reported by Harrison et al, ranged from 1.4 to 2.6. Thus, sample variability raises issues with the predictability of accuracy in exposure estimation.21 This variability may also skew modelling as well, resulting in fallacious interpretations; although as mentioned in Harrison et al, when the population sample becomes larger or uses pooled data these influences may become diminished.
Historically, most inferred that there is no relation between personal and static exposures,2–4,6,9–11 while studies such as that performed by Harrison et al, question this concept. Establishment of a relation between these two sampling methods will allow incorporation of additional data into occupational, environmental, and epidemiological studies,16 although caution must be applied in interpreting any relation based on previous findings.2,4 Thus, care must be exercised when evaluating studies that solely use static sampling as the method of estimating personal exposure.7