59 Not surprisingly different sampling techniques are needed for each of the
above states of matter.
6.1.1 Sampling Techniques
The fundamental requirement of any measurement technique is that it should be appropriate for the purpose of the measurement. This means it should
provide information necessary for the decisions which will be made on the basis of that information.
„Monitoring‟ or “Sampling” means the use of valid and suitable occupational hygiene techniques to derive a quantitative estimate of the exposure of
employees to substances hazardous to health. Only validated monitoring methods should be used, these are published by organisations such as the
HSE in the UK and NIOSH in the USA. Other countries also produce methods and in some cases these are specified as being compulsory within local
legislation. In the case of airborne contaminants monitoring involves the periodic or continuous sampling of the atmosphere at the workplace and will
usually require sampling in the breathing zone of the operative by means of personal sampling equipment.
In addition to personal monitoring, fixed positionstatic monitoring can also offer limited information as to an individual‟s exposure. However, it can
provide a guide to the sources of contaminants, effectiveness of control
measures and the general workroom atmospheric concentrations.
6.1.2 Types of Sampling
There are five main types of sampling:
Grab
Grab or snap sampling can be used as a screening technique; it will give the concentration of a contaminant at a specific time and location and will help to
confirm the presence of andor identify a suspected contaminant.
60 C
o n
c e
n tr
a ti
o n
Time
Source: Adrian Hirst
Short term
Short term monitoring will determine concentrations over a short time period, normally up to 10 or 15 minutes. Results are normally calculated as a time-
weighted average TWA and can be compared with any relevant recommended short term exposure limits especially WELs and to quantify
exposures to acute hazards e.g. cadmium.
C o
n ce
n tr
a ti
o n
Time
Source: Adrian Hirst
Long term
Long term monitoring is similarly determined on a time-weighted average basis and related to long term 8 hour TWA recommended limits. Half Shift 4
hours, or the time to complete a specific operation or full shift nominally 8 hours and normally the time periods monitored.
61 C
o n
ce n
tr a
ti o
n
Time
Source: Adrian Hirst
Continuous
Monitoring will indicate the variations in concentrations and is similar to grab sampling in that peak levels can be identified as well as the average
concentrationsexposures being determined.
C o
n c
e n
tr a
ti o
n
Time
Source: Adrian Hirst
Bulk
In some circumstances, bulk samples of the substances being handled may also be taken and analysed for identification purposes, but it is not possible to
relate the results from bulk samples to the airborne samples collected. However, with some contaminants, such as asbestos, bulk sampling is an
essential part of the identification process.
62 The sampling types described above and the exposure limits quoted in EH4O
in the UK or other legislationdirectives are based on the assumption that inhalation is the main route of entry into the body. However, skin absorption
and ingestion can also occur and the only sure way of measuring exposure to substances entering the body by these routes is to apply biological monitoring
methods. Usually, such methods measure the amount of a substance or of one or more of its metabolites in one or other of the two accessible body fluids
– blood or urine. Details of such sampling techniques will be covered in Section 8 of this course manual.
Measurements for particular substances at a particular time tell only part of the story, and it should be remembered that concentration can vary as
process changes etc, occur. Proper sampling strategies must be adopted to decide which groups of workers, which plant locations and which shifts should
be monitored. One of the first questions you should ask about of any sampling technique is
what will the results tell me. What are the norms against which they may be judged?
6.2 Sampling Equipment