User`s guide

Sampling Techniques
332
obtained from regular monitoring for individual analytes will vary
considerably.
Figure 99 Probability of a Result Occurring versus Air
Concentration
Due to the wide spread of results expected, at least 10 results,
preferably 20, are frequently required before a true average air
concentration or exposure level can be established.
This is illustrated by the following set of data obtained during a field
monitoring study of a group of individuals at a petrochemical plant.
All individuals are performing the same job.
Table 27 Field Monitoring Data
Once an accurate average concentration has been determined, it is
compared with the relevant limit level. However, because of the
spread of the monitoring results, it is not sufficient to show that the
mean is less than the legal maximum.
Data (ppm) 3.0 4.1 4.2 0.7
33.0 12.0 0.47 10.0
1.7 17.0 4.2 19.0
4.4 6.2 7.9 2.2
1.3 2.5 1.4 2.3
Samples 20
Mean Value 6.9 ppm
Range 0.47 to 33.0 ppm
Probability of
Result Occurring
Air Concentration
(
lo
g
.
)
3.33 5.77 10 17.32 30