Up close and personal

So what is a safe level? The Health & Safety Executive publication EH40 lists the occupational exposure limits for hazardous substances at work and publishes tables of figures to allow you to identify the long-term exposure limit (eight hour TWA reference period) and short-term exposure limit (15 minute reference period).

The simplest and least expensive method of measuring these levels is to use a chemical stain tube. Using a simple hand or electronic pump, a fixed volume sample of gas is drawn through the tube. The gas tube incorporates an accurate etched marking to illustrate the gas measurement. A colour change takes place, and the length of the stain translates into a measurement for the gas in question. This method has proved popular for many years because it is a low-cost, simple technology and many different tube types are available for different compounds and ranges. With spot checking gas tubes at just £1 each, and the hand pump at £79, this method will continue to be popular.

But spot-checking does not give the complete picture. What if workers’ exposure levels vary continuously? One spot check could show a high or low reading. You need to take numerous spot readings to arrive at reliable 15 minute and eight hour average exposures.

An alternative is the long-term tube. It uses a battery-powered pump or passive colour change badges. More commonly for 15 minute or eight hour sampling, a sorbent tube and battery operated pump can be used for many gases and vapours. The sorbent tube normally contains two layers of a sorbent material (charcoal for example). Air is pulled through the tube and the solvents are collected on the first layer. There is also a back-up layer in case the first one fails. After sampling, the sorbent tube requires laboratory analysis. Trapped chemicals are extracted and details of chemicals and the quantity of each is also identified using gas chromatography.

This technique is accurate but there is a potential for errors if work is done by unskilled staff. The use of a pump, a sorbent tube, the time delay in sending the tube to the lab, and the cost of analysis makes it far from ideal.

Also, although the measurement is considering a full working period of eight hours, the result obtained is only the average of the work period. The exposure could have been as a result of a high concentration for a short time and low or zero exposure for the rest of the day. You cannot tell the profile of exposure throughout the day from an eight hour sorbent tube sample, so you are unable to accurately identify if adjusting a worker’s activity will minimise exposure.

Recently introduced to the industry, the small and lightweight personal VOC monitor provides an immediate warning of high concentrations. The monitor incorporates a clip to connect the monitor to the lapel of the worker to allow them to wear it and forget it. But in the event of a high level of gas, it immediately sounds an alarm and a flashing light attracts attention. The worker can take corrective action or retire to a safe area.

The personal monitor measures a range of VOCs and displays the total on its digital display. The range includes toluene, MEK and trichloroethylene. The VOC monitor is a Photoionisation Detector (PID). A PID uses an ultraviolet light to break down chemicals to positive and negative ions that can easily be measured with a detector. This detector measures the charge of the ionised gas and converts the signal into current. The current is amplified and displayed on the meter as parts per million.

Correction factors are an important tool in the use of PIDs. They permit calibration on one gas while directly reading the concentration of another. This eliminates the need for multiple calibration gases.

PID manufacturers determine correction factors by measuring a PID’s response to a known concentration of target gas. Correction factors tend to be instrument, and/or manufacturer specific, so it is best to use the correction factors given by the PID manufacturer.

Menu of compounds

To make it easier to work with, users can select the compound they are interested in from a menu of compounds built into the instrument. Having selected the compound, the correction factor is automatically applied.

While the monitor provides an immediate warning to the worker, it will also allow the manager to identify a profile of the workers’ exposure through its datalogging capability. With capacity for datalogging over an eight hour period, data is easily sent to a PC in report and graphical format. This allows managers to pinpoint high concentrations and consultation with workers enables them to identify why peaks occurred.

Action can thus be taken to avoid these high exposure periods. The worker could have turned off the ventilation for a short period. The data record would immediately highlight an increased level of exposure. Individual workers may possibly clown around and deliberately expose themselves to high levels. A graphical record showing a severe, high spike would give rise to suspicion of unacceptable working practices.

The user may programme alarm levels. With reference to EH40, alarms can be set for an eight hour TWA period, 15 minute short-term exposure period and an instantaneous high level.

Unlike adsorptive techniques, where the worker is exposed

and you do not know until it is

too late, the VOC monitor tells

you immediately, and records the details digitally, whenever there is a problem.