Methodologies for testing airborne contaminants

There are a number of substances that can have a hazardous effect on health as a result of being breathed in. In this article, Lisa Bushby describes several established techniques to ensure that the concentrations of airborne contaminants — of the type that may cause respiratory disorders — remain below established workplace exposure limits.

Introduction

Workplace Exposure Limits (WELs) are concentrations of airborne hazardous substances, averaged over either 8 hours (long-term) or 15 minutes (short-term) and have been established to protect the health of workers.

The 8-hour Time-Weighted Average (TWA), expressed in units of ppm or mg/m3 may be determined by:

where C1 is the occupational exposure and T1 is the associated exposure time in hours in any 24-hour period.

For the short-term reference period, Exposure should be recorded as the average over a period, normally 15 minutes, and should be determined by sampling over that period.

Some limits are statutory, meaning concentrations of the substances they relate to may not be exceeded. These statutory limits are published in the Health and Safety Executive’s (HSE) EH40 document, although it should be noted that the absence of a substance from EH40 does not necessarily indicate that it is safe. Exposure to any substance should always be controlled to a level to which nearly all the working population could be exposed without any adverse effects on health.

GESTIS is a database that contains validated lists of methods from various EU Member States described as suitable for the analysis of chemical agents at workplaces, including analytical methods for hazardous substances in the workplace atmosphere. The GESTIS substance database can be directly accessed via the Institut für Arbeitsschutz der Deutschen Gesetzlichen Unfallversicherung (IFA) website — see here. However, where no method exists — eg in the case of a novel substance, it may be necessary to develop and validate a method in-house. Many of the techniques for specific substances are described in the HSEs Methods for the determination of hazardous substances (MDHS) series.

Sampler positioning

While most WELs refer to personal exposures, which require “personal monitoring” where the sampler is placed within 20–30cm of the employee’s breathing zone (typically on the lapel), fixed place or static monitoring may also be used:

  • to obtain information on the likely sources contributing to the exposure

  • to check the effectiveness of control measures

  • to determine background workplace contaminant concentrations

  • if there are no personal monitoring methods available.

Let’s consider the established techniques for testing airborne concentrations of aerosols and dusts, commonly associated with inhalation disorders — and then look at examples of testing methodologies for crystalline silica, isocyanates and cadmium – each associated with specific disorders.

Respirable, thoracic and inhalable aerosols

The fractions of any suspension of particle in air (an aerosol) can be described as:

  • inhalable — fraction of airborne material that enters the nose and mouth during breathing, and is therefore available for deposition anywhere in the respiratory tract

  • thoracic — fraction penetrating beyond the larynx

  • respirable — fraction that penetrates to the lower gas exchange region of the lung.

The behaviour, deposition and fate of the particle will be determined by its chemical nature and the size of the particle. It is important to consider both the concentration and size of fractions present. The general methods for sampling and gravimetric analysis of respirable, thoracic and inhalable aerosols are described in MDHS14/4, although it should be noted that the use of alternative methods is acceptable provided the accuracy and reliability appropriate to the application can be demonstrated.

The procedure describes the analysis of the collected aerosol using the gravimetric technique. After drawing a measured volume of air through the pre-weighed collection medium (eg filter or foam) mounted in a suitable particle size-selective sampler, the mass concentration can then be determined from the mass of the aerosol collected and the sampled air volume.

Dusts

Dusts are responsible for a wide range of inhalation disorder.

The presence of many different types of particles, both solids (eg dusts, fumes and fibres) and liquids (organic or inorganic mists) can be revealed using a dust lamp (MDHS82) (a simple tool for observing the presence of airborne particles), although the technique does not give a quantitative measure of either particle concentration or size.

The main use of the dust lamp is to make fine (<10µm) airborne particles visible, although it can also enhance the visibility of dust clouds containing coarser particles.

For fine particles, the intensity of the scattered light is greatest at a small angle to the incident light beam. As the angle is increased, the intensity of the scattered light falls rapidly, consistent with diffraction theory.

Crystalline silica

Crystalline silica may cause silicosis and lung cancer. As described in the publication EH40, the WEL of respirable crystalline silica is 0.1mg/m3. If exposure cannot be controlled to this level (or below it) by elimination or process or engineering controls, exposure must be controlled through the provision and use of suitable respiratory protective equipment. Airborne concentrations may be determined by the direct-on-filter analysis by infrared spectroscopy and X-ray diffraction (MDHS101). Essentially, a sample of respirable dust is collected on a membrane filter using a respirable dust sampler (most commonly a Higgins-Dewell cyclone sampler with a 25mm diameter filter). The filter is then placed directly into the sample beam of either an infrared spectrophotometer or an X-ray diffractometer. The mass of crystalline silica on the filter is determined from the infrared or X-ray diffraction response, calibrated against filters loaded with known amounts of standard quartz or cristobalite. Since the volume of air sampled will be known, the concentration of airborne crystalline silica is readily calculated.

Organic isocyanates

Isocyanates may cause occupational asthma. As described in EH40, the WEL of all isocyanates (other than methyl isocyanate) is 0.02mg/m3.

Airborne concentrations are typically determined (MDHS25/3) by a laboratory method using sampling either onto 1-(2-methoxyphenyl) piperazine (1-2MP) coated glass fibre filters followed by solvent desorption or into impingers and analysis using high performance liquid chromatography (HPLC). The method can be used to determine TWA concentrations in workplace atmospheres and is suitable for sampling over periods in the range of 15 minutes to 8 hours.

A measured volume of air is drawn through a glass impinger containing absorbing solution and/or a filter impregnated with the appropriate 1-2MP solution. Any organic isocyanates present will react to form non-volatile urea derivatives. The resultant solution is concentrated and analysed by HPLC with UV and electrochemical (EC) detection. Isocyanate-derived peaks are identified on the basis of their EC/UV responses and also by diode array detection and comparison with derivatised bulk. Quantification is by comparison with the relevant isocyanate monomer standard. Total isocyanate in air concentration is calculated from the sum of all the isocyanate derived peaks.

Cadmium

Exposure to airborne cadmium and inorganic compounds of cadmium may lead to chronic obstructive pulmonary disease. Cadmium and cadmium compounds (except cadmium oxide fume, cadmium sulphide and cadmium sulphide pigments) have a WEL of 0.025mg/m3.

MDHS10/2 describes laboratory testing using flame atomic absorption spectrometry or electrothermal atomic absorption spectrometry.

A measured volume of air is drawn through a filter mounted in an inhalable or respirable dust sampler, as appropriate. The filter and collected sample are then treated with 5ml of 1:1 nitric acid and heated on a hotplate until about 1ml of the concentrated nitric acid solution remains. This is diluted to 10ml with water and the resultant solution is analysed for cadmium by aspirating into the oxidising air-acetylene flame of an atomic absorption spectrometer. Absorbance measurements are made at 228.8nm. For accurate determination when the concentration of cadmium in the solution is low, the analysis may be repeated using electrothermal atomic absorption spectrometry.