Sampling the air
February 1st 2007

Among the many potential dangers that may be present in the workplace are airborne hazards from gases, vapours and dusts, many of which can cause potentially life threatening illnesses. Air sampling can help ensure that employees are protected from such hazards, as Eddie Salter of SKC UK explains

Air sampling is relevant to almost every industry, from flour dust exposure in a bakery to chemical vapour exposure in a factory. By consistently measuring the concentration of airborne contaminants workers are exposed to, steps can be taken to ensure they remain at a safe level, preventing chronic respiratory diseases such as Asthma and Pulmonary Fibrosis.

Put simply, air sampling is capturing the contaminants in a known volume of air and measuring those contaminants as a concentration. If you are unsure about how to go about starting your sampling regime, here are five simple questions to ask: Who are the most susceptible? Which workers are most likely to come into contact with contaminants? Where are the most susceptible? For example, in which department are they located? When are they most likely to be exposed, i.e. at what stage of the manufacturing process are they most likely to be exposed? What are you looking for? If you do not know which contaminant you are looking for, you won't be able to capture and measure it effectively.

How do you do it? The Methods for Detection of Hazardous Substances (MDHS) are available from the HSE (www.hse.gov.uk).

A widely used method is to connect a battery-operated pump worn on a belt to a collecting media positioned in the breathing zone (within a 10cm radius of the nose and mouth area). The pump should be capable of drawing air through the media at a constant rate for more than 8h, even in adverse conditions such as extreme temperatures. The contaminant is then captured on the media for analysis. Media can be a filter for dusts and particles or a sorbent tube such as charcoal for gases and vapours.

There are two types of contaminant, grouped according to their physical properties: dust and particles, and gases and vapours. Particulates can be further divided into five types, defined as follows: Aerosol – dispersion in air of microscopic particles of solid or liquid; Dust – solid particulate capable of temporary suspension in air; Fume – solid particles produced by condensation from the gaseous phase; Smoke – particles resulting from incomplete combustion of organic matter; Mist – dispersion of droplets in air.

Once measured, the contaminants are expressed as milligrams per cubic metre (mg/m3) for particulates and Parts Per Million (PPM) for gases. For example, 1PPM of a gas means that 1 unit of the gas is present for every 1 million units of air. The maximum exposure limit (MEL) is the maximum concentration of an airborne substance to which employees may be exposed by inhalation over a set period of time. Measuring in this way ensures that these limits are not being exceeded.

Calibration Calibration of the airflow through the sampler is important and should be checked before AND after every sample is taken. Broadly speaking, there are two different levels of flow commonly used for personal sampling systems – around 2L/min for dust sampling and between 10 and 200mL/min for gases and vapours.

The usual method of flow measurement (calibration) for the higher flow rates has generally been via a Rotameter and, for the lower flows, a bubble film calibrator has traditionally been used. There is now a third option which is to use an electronic calibrator, such as the Defender series, which is fast, highly accurate and provides NIST traceable readings.

If you have never carried out air sampling before, or you are unsure of the safe exposure limits, help is available from organisations such as the Health & Safety Executive.

A range of courses are available which offer basic training on the use of air sampling equipment. These are designed to help you get the most from your sampling routine so that you can be sure your employees are working in an environment where airborne contaminants remain at a safe level.

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