Measurement and Monitoring Strategy for Assessing Workplace Exposure to Airborne Nanomaterials - Safety of Nanomaterials along Their Lifecycle - page 236

What is the average background concentration taking into account the mea-
surement characteristics of the measurement procedure involved (e.g., time
characteristics) and what is its standard deviation?
Are defined sources of emissions identifiable/locatable (above background
level)? If yes, what are the average emission concentration and its standard
deviation?
Are nanoobjects in the breathing zone of workers (exposure concentrations)
identifiable (above background level)? If yes, what are the average exposure
concentration and its standard deviation?
Are the identified exposure or emission concentrations significantly above back-
ground level?
For the background determination, it has to be taken into account that other pro-
cess-related particle sources, such as electrical motors or hot surfaces, may emit par-
ticles and therefore contribute to the particle concentration in the workplace. Often
these particles are in a similar size range as the nanomaterial produced or handled
and are therefore included in the particle concentration reading of the handheld
particle concentration monitor. Ideally, the background is hence measured prior to
the process step with all equipment and machinery running, only without the nano-
material being applied. This is, however, often impossible. Instead, the background
concentration can be measured either prior to and after the process step under inves-
tigation or during the investigated process step at a representative background site.
If the workplace under consideration is enclosed and has a forced ventilation system,
the background can best be measured by assessing the particle concentration directly
in the supply air. The advantage of a parallel measurement of the background is that
potential influences from outside (fork-lift, welding, smoking, etc.) can be excluded
from the determination. This type of far-field measurement does, however, request
the availability of a second measurement instrument.
The background determination measurements have to cover a time period of typi-
cally at least 45 min. During this period several measurements of equal lengths have
to be performed (e.g., 9 times 5 min or 3 times 15 min). The results of these individ-
ual measurements have to be recorded, documented, and their arithmetic means as
well as standard deviations have to be calculated and documented. The time series of
the concentration measurements also needs to be taken into account and be critically
evaluated, because they may reveal only short-lived particle releases which, depend-
ing on the local flow situation, can lead to very short spikes in the measured particle
concentration. Depending on the averaging time, such short spikes may completely
vanish and only have a small impact on the standard deviation. In such cases, the
averaging times need to be lowered.
If a source of possible nanomaterial emissions could be identified, the concentra-
tion near that source has to be determined during application of the relevant process
and the reason why this particular source of emission is considered relevant has to be
documented. The measurements are repeated several times to cover the same mea-
surement intervals as during the background determination (see the aforementioned).
If the emission process does not last for such a time period, the complete duration
of the process has to be covered as a minimum. Again, the arithmetic mean and