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| Module 1: Occupational Hygiene - Section 1: Introduction to Occupational Hygiene |
| OH1.5:Environmental Monitoring |
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| Module 1: Occupational Hygiene - Section 1: Introduction to Occupational Hygiene |
| OH1.5:Environmental Monitoring |
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The assessment of airborne concentration of a particular substance and comparison with the appropriate standard is usually the primary technique in the evaluation of the working environment. There are two approaches that can be used to evaluate health hazards, viz. environmental monitoring and medical surveillance. Environmental monitoring assesses the level of airborne contaminants in the workplace atmosphere. Medical surveillance evaluates absorption of hazardous substances into the body and their early and late effects on health.
What is air sampling?
Air sampling is taking a known volume of air through a medium (normally a paper or other filter for solids and a sorbent for gases), on which the contaminant is captured. The concentration of the contaminant in air is calculated using the volume of air passed through the collection device and the amount of pollutant captured. These are discussed in detail in following sections.
The purpose of air monitoring is to measure the level of airborne concentration of chemical hazards to which an employee is exposed. Results of air monitoring are also sometimes used to evaluate the effectiveness of engineering measures in controlling worker's exposure to airborne contaminants.
Worker selection
Before conducting air monitoring, exposed workers should be divided into homogenous exposure groups Occupational Exposure Sampling Strategy Manual (OESSM): Section 3.* This means that job tasks are analysed in relation to exposure profile and all workers who are similarly exposed are grouped together. The worker with the highest exposure in a group is then selected for monitoring.
The rationale for selecting a maximally exposed worker is that whatever recommendations regarding that worker will automatically apply to all other workers in that group because they perform the same or similar tasks. This is a conservative preventive approach which errs on the side of caution
If it is not possible to identify a worker with the highest exposure, then the Approved Inspection Authority should consult the (OESSM) Section 3 * The OESSM strategy is used to identify the number of workers in an exposure group that have to monitored in order to be 95% certain that the representative sample includes the highest exposed worker.
Sample type
Contaminants in the workplace are not uniformly distributed throughout the workplace. They tend to concentrate around sources of emission, the concentration falling off rapidly with distance.
The only way to measure what a person is exposed to is to position the sampler in the person's breathing zone. This is defined as within 30 cm of nose and mouth. Because it is not possible to place the sampler on or in someone's nose, it is usually attached to the lapel (collar of shirt) as high up as possible. Measurements made in this way are called personal (breathing zone) samples.
The alternative to personal sampling is static sampling where the position of the sampler is fixed next to a workstation. It is sometimes called area sampling. This may be a useful measure for testing effectiveness of control measures in a specific location in the workplace.
Sampling methods
There are many different methods of taking air samples, but by far the most widely used is the sampling train method *. It consists of a battery-operated person sampling pump, a connecting tube, and a sampling medium.
Airborne chemical substances are measured using an air sampling pump which passes air, at a pre-determined rate, through a medium which traps the contaminants. The "trap" may be either a charcoal tube, a filter paper or a container, filled with distilled water or a chemical reagent. The pump should be capable of drawing air through the medium at a constant for at least 8 hours.
A charcoal tube is a glass tube that contains activated carbon which catches the molecules of gases or vapours onto its surface, as air is pulled along its length. After sampling the tube is sealed and sent to a laboratory for analysis. Analysis entails determining the amount of contaminant that is trapped on the surface. The trapped contaminants are released by solvent washing or heat and analysed using specialised equipment. Charcoal tubes are ideal for sampling gases and vapours.
The filter paper can be analysed chemically or can be weighed to determine the amount of contaminant deposited. For gravimetric analysis the filter is weighed before and after sampling and the difference is the amount of contaminant collected. The filter can be dissolved in a special reagent and a specialised method is used to find out how much of the contaminant is dissolved in the solution. Filter paper is used to sample air for dust, fumes and mist.
Air is bubbled through a chemical solution contained in a glass impinger or bubbler for a predetermined period. The contaminant contained in air reacts with the solution resulting in either a colour change, or a solid substance (precipitation) is deposited in the tube. The intensity of the colour change or the amount of precipitation gives an indication of the amount of contaminant that is present in the workplace atmosphere. This method of sample collection is ideal for reactive gases and vapours.
Flow Rate
Broadly speaking there are two different levels of flow commonly used for personal sampling systems. For dust, the flow is set at around 2 litres of air per minute, and for gases between 10 and 500 millilitres per minute.
Sampling duration
Depending on the nature of the health effect of the contaminant being monitored, workplace atmosphere can be sampled for a short period or over a longer period. For substances that have acute health effects the sampling duration varies between 10 and 15 minutes. Chemical substances that have chronic health effect are monitored over a longer period,around 7 or 8 hours. The longer you take a sample for, the greater the degree of reliability. For legal compliance samples should be taken over the entire shift (a full working day's exposure).
[The module on air sampling and analysis provides a more detailed discussion of the various techniques and equipment used to evaluate airborne hazardous substances]
* US Government - Public Domain
