Personal exposure monitoring is often conducted for hazardous substances such as wood dust and solvent fumes – as well as noise. The aim is to identify the level of exposure for individuals and evaluate this level against the relevant legal limits. If exposure levels are not as low as reasonably practicable, a plan should be developed to improve the situation.
Unfortunately, it is common to find sites that have had monitoring conducted on their behalf but have done nothing with the results – as they think that they have fulfilled their duties by hiring the monitoring consultants. As with more general risk assessments, the presence of documents does not fulfil a company’s legal duties if no improvement action has been taken where the results have indicated that it is needed.The Control of Substances Hazardous to Health Regulations 1999 (COSHH) apply to hazardous substances such as wood dust and solvents. The regulations require that all such substances are identified, evaluated and controlled so as to minimise the levels of personal exposure – as far as reasonably practicable.
Complying with the regulations will involve a number of steps. First, it is necessary to identify substances that may be hazardous to health. These should be measured and evaluated to allow an assessment to be made of the risk to health. The adequacy of control measures should be evaluated and improvements should be identified and implemented.
Monitoring is not an explicit requirement of the regulations. However, quantified Occupational Exposure Limits (OELs) exist for many substances and it is often difficult to say with any certainty whether exposure is well below these limits if there has been no measurement.
Two types of limit are of interest under COSHH:
Wood dust (both hard and soft) has an MEL of 5mg/m³. An individual’s level of exposure is typically measured by fixing a scientific pump on them. This draws a known volume of air through a filter. The pump will typically be set at 2 litres per minute and should be left attached for a representative period. The longer the monitoring period, the more representative the result.
The filter is fixed within 30cm of the nose-mouth region of the person being monitored. The filter is weighed before and after the monitoring exercise – with the difference in weight being attributed to the wood dust. The weight increase over the monitoring period is adjusted to provide a figure in cubic metres ie 1,000 litres – which can be compared with the MEL.
It should be noted that the 5mg/m³limit is not a simple on/off switch, with 4.9mg/m³being acceptable. As a rough guide, a result of less than 0.25 of a limit can be deemed satisfactory, 0.25-0.5 requires further investigation and more than 0.5 requires further monitoring along with detailed investigations.
Therefore, if the result of the wood dust monitoring is 2.5mg/m³or more, further dust reduction measures will be needed. Recurring suggestions to minimise wood dust exposure include:
Monitoring solvents
A wide range of solvents is used in the timber and furniture industries in adhesive, wood treatment and wood coating. Many solvents have occupational exposure standards, for example, toluene has a short-term exposure limit (15 minutes) of 150ppm and a long-term limit (8 hours) of 50ppm.
When monitoring for solvents, an important first step is to decide what to monitor. By way of illustration, a wood coating operation may use 100 different coatings, each of which comprises a variety of solvents. Typically, 95 of the coatings will be used only in minor quantities, so it will be necessary to concentrate on the main five coatings and review their major constituents.
For example, a precatalysed wood coating might contain: xylene 10-25%, acetone 10-25%, butyl acetate10-25%, ethanol 1-10% and methanol 1-10%. Therefore, monitoring might concentrate upon xylene, acetone and butyl acetate especially if they were major constitutents of the other main wood coatings.
A cheap and easy way to monitor exposure is through the use of detector tubes. Many varieties are available, with a different tube required for each type of solvent. Direct reading detector tubes are used with a volumetric pump which draws a known volume of air (typically in multiples of 100ml) through the detector tubes.
Each tube contains a reagent which reacts with the specific solvent, producing a coloured stain. The greater the amount of solvent in the air, the longer the stain. The concentration can then be calculated from a scale on the side of the tube.
This method has the advantage of giving a quick indication of solvent concentrations. However, tubes typically have an accuracy of +/- 25% to 35%, so the method is often used as an initial method to determine whether more accurate readings are required.
The alternative type of detector tube does not require a pump. Passive dosimeter tubes work via the natural diffusion of solvents in the air. Tubes are left exposed for 1-8 hours and an instant reading is provided. The accuracy of the reading is related to the number of hours that the tube is exposed.
If monitoring results show that action is required, typical areas to concentrate upon include:
It should also be noted that inhalation is not the only concern with regard to solvents. Some can be absorbed through the skin, so gloves should be used when handling them. Such information should be made clear on the relevant data sheet.
Noise levels
The Noise at Work Regulations 1989 require employers to reduce the risk of hearing damage to the lowest level reasonably practicable. Where levels of personal exposure are likely to exceed 85dB(A), a noise survey will be required.
The survey will typically have two parts. The first will involve the construction of a noise map which shows the noise levels in all parts of the factory. Once the map is compiled, it is possible to identify those that are likely to be exposed to noise at or above the first action level of 85dB(A). Dosimeters can be attached to these individuals to obtain a reading for personal exposure levels.
The preferred option for noise reduction is prevention at source, for example, through the proper specification and maintenance of equipment. Consideration should then be given to attenuation, ie reducing the transmission of noise before it can reach the worker. This may involve blanketing walls, ceilings and partitions with noise absorbing materials. Personal protective equipment is the last resort to control noise.
