Dampness and Mould - WHO guidelines for indoor air quality - PRWeb

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WHO GUIDELINES FOR INDOOR AIR QUALITY: DAMPNESS AND MOULD
2.4.4.2. How useful are routinely collected data?
Data collected for use in monitoring may be of limited value in epidemiological
studies. For example, monitoring is often done in areas where the concentrations
are likely to be highest, to ensure compliance with exposure limits. In contrast,
epidemiological studies require information on average concentrations. Special
surveys may therefore be necessary, with random sampling, rather than relying
on data collected during monitoring.
2.4.4.3. When should sampling be done?
To the extent possible, samples should be taken so that they represent the true
exposure at an appropriate time. For acute effects, exposure measured shortly
before the effects occur is the most useful. The situation is more complicated
for chronic effects, as, ideally, exposure should be assessed before the effects occur
and preferably at the time they are biologically most relevant – that is, when
the exposure is considered the most problematic or when people are most likely
to be exposed. This is possible only in prospective cohort studies or in retrospective
cohort studies in which information on past exposure is available; even
then, it is often unclear when people are most likely to be exposed to the agent
of interest. In cross-sectional studies, exposure measurement can be valuable for
assessing past exposure, but only when the environment has not changed significantly.
2.4.4.4. How many samples should be taken?
Measures of exposure should be sufficiently accurate and precise that the effect
on disease can be estimated with minimal bias and maximum efficiency. Precision
can be gained (i.e. measurement error can be reduced) by increasing the
number of samples taken, either by increasing the number of people for whom
exposure is measured or by increasing the number of measurements per person.
In population studies, repeated sampling is particularly effective for exposures
known to vary more widely over time within people than among people. If the
within-people variation is lower than that between people, repeated measurements
will not reduce the measurement error significantly. If there is known
within- and between-person variation (from previous surveys or pilot studies,
for example), the number of samples required to reduce bias in the risk estimate
by a specific amount can be computed in the manner described by Cochran
(1968) (see also section 2.4.5).
2.4.4.5. Should settled dust or airborne samples be taken?
In many studies, reservoir dust from carpets or mattresses is collected, and the
concentrations are usually expressed in either weight per gram of sampled dust
or weight per square metre. Although both measures are generally accepted,
the latter may better reflect actual exposure (Institute of Medicine, 2004). The