Evaluation of Airborne Contaminants

Module 1: Occupational Hygiene - Section 3: Evaluation of Airborne Contaminants

OH3.2: Sampling Strategy

SAMPLING STRATEGY

To accurately measure airborne contaminant levels, a sampling strategy* must be devised. Preparation of a sampling strategy needs to take into account the following areas:

Where to sample?
Whom to sample?
How long to sample for?
How many samples?
When to sample?

Sometimes sampling, measuring and analysis are used confusingly or interchangeably.

It should be remembered that there must first be a sampling strategy which determines where to sample and whom to sample in terms of exposed workers. Once these questions have been settled, individual workers or locations in the workplace are chosen to have measurements taken of the hazardous exposure of interest. After this measurement process, the material that has been collected in the measurement device is then quantitatively and qualitatively analysed in the laboratory.

Where to sample?

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.

Air sampling can be done either by way of personal or static monitoring.

Personal sampling is the preferred method to determine a worker's exposure. In this method the air sample is collected within the breathing zone of worker. The breathing zone is defined as being within a 30 cm radius of nose and mouth. Because it is not possible to place a sampler on someone's nose, it is usually attached to the lapel (collar of shirt) as high up as possible. Personal samples ensure as far as possible that the air sampled best represents the air inhaled by the worker.

The alternative to personal sampling is static (area) sampling where the position of the sampler is fixed next to the workstation or source of contaminant. This does not give a good estimate of a worker's exposure and is of limited use in assessing the risks posed to the worker. Static sampling is ideal for testing effectiveness of control measures.

Whom to sample?

It is generally not feasible to sample all workers over a period of time. To minimize sampling and analytic costs, and to increase effectiveness of sampling programmes, a subset of employees is selected and their results are used to indicate exposure conditions for the rest of the workforce.

Which workers to sample will depend on the number of job specific areas or workstations and the time spent in these locations. Before sampling, potentially exposed groups should be divided into homogenous exposure groups (heg). This means that tasks are analysed in relation to their exposure profile and all workers who are similarly exposed are grouped together. Workers who have the greatest potential for exposure should be included in the sampling frame (worst case sampling). The rationale for selecting a maximally exposed worker is that whatever recommendations regarding that worker will automatically apply to all other workers in the similarly exposed group because they perform the same or similar tasks.

If it is not possible to identify a worker with the highest exposure then the NIOSH Occupational Exposure Sampling Strategy Manual (OESSM)* should be consulted. The OESSM strategy is used to identify the number of workers in an exposure group that have to be monitored in order to be 90 or 95 percent certain that the representative sample includes the highest exposed worker.

How long to sample for?

Depending on the nature of the hazardous substance or health effect, either short term or long term may be required. The length of time sampling is undertaken should be sufficient allow a representative level to be obtained.

The average sampling time can vary from as little as 10 minutes to the full 8-hour shift, depending on the exposure standard. Hazardous substances that have chronic health effect are monitored over a period of 7 to 8 hours. For substances that have acute health effect the sampling duration varies between 10 and 15 minutes.

Other factors to consider when deciding on sampling duration are:

Sensitivity of the analytic methodology
Whether on needs to estimation of the concentration of the substance present; and
Regulatory requirements.

When to sample?

Initially, more frequent sampling may be required to establish base line-levels. Seasonal variations between temperature, air movement and humidity, and shift and production timetable, can all affect levels. Therefore, any sampling strategy should include sampling during these times. This goes to the need to obtain representative levels of contaminants to which workers are exposed on average - sampling should encompass all aspects of the job where possible.

Where exposures exceed the exposure limits, controls will be need to be applied in hierarchical fashion, and more frequent follow-up monitoring will be required to assess the efficiency of the control methods used.

* US Government - Public Domain

General Introduction to Occupational Health: Occupational Hygiene, Epidemiology & Biostatistics by Prof Jonny Myers is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.5 South Africa License.