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Basic concepts in hazardous chemicals: monitoring toxicity

By Wan Zainal Azman Wan Abdullah
The Sun, April 4, 1996

The word chemical often conjures up images of damage, debility, disease, and death in the minds of men. These dangers are all too real and must be understood and anticipated to avoid unnecessary ill health and loss of life.

We should take appropriate steps to minimals exposure to toxic chemicals. One area of the environment where we can and must control exposure to toxic hazards is the workplace.

At the workplace, a hazardous chemical may be present in any one of several forms, be it dust, mist, fume, gas, vapour, liquid, or solid.

The safe use of chemicals requires an understanding of the agents involved, including their toxicity, methods of application, environmental fate and numerous other factors. Consideration must also be given to every facet that will influence the effect of these agents on humans, animals, plants and other elements in the environment.

A good starting point in the effort to avoid chemical toxicity is the identification of all chemicals within the area of exposure.

This involves the evaluation of systems, industries, processes and other activities wihtin the area and the determination of whether any of them pose a potential health risk. If the risk assessment of a particular compound is sufficiently extensive and accurate, it can prevent injury and ill-health.

There are well-established principles of toxicology and occupational health that apply to all compounds and that have been found to be useful for keeping injuries in the workplace to a minimum.

Injury may be more efficiently prevented if there is continuous monitoring of the exposure of workers. With the problem defined, exposure may be maintained at a safe level through education and protective devices and practices, which may include appropriate design and choice of active ingredients and formulations of a particular chemical.

Also, regulations can define the different measures that help to prevent injury and can ensure that the measures are carried out effectively.

Monitoring of exposures

Real-time monitoring involves analytical techniques that can instantly detect the presence of a chemical and alert the population in the affected area. Continuous real-time monitoring involves a device that can be used constantly during the period of potential use or exposure to that agent.

One form of real-time monitoring utilises colorimetric tubes for detection and quantitation of materials. A calibrated battery or hand-powered pump is used to draw air through the tube. The matrix (air, water, soil, etc) in the tube is coated with chemicals that react with the contaminant in the air and produce a color indicative of its presence.

Colorimetric tubes vary in sensitivity, specificity, accuracy and precision. In addition, tubes may be specifically designed for gases and vapours, and therefore may provide inadequate response for aerosols.

Although it is relatively easy to use these devices, it is also easy for untrained people to overlook the important nuances of their proper use and interpretation.

These devices are useful for determining an approximate concentration of a chemical. However, they have inherent inaccuracies that more experienced personnel will appreciate. They are useful as a screening method to determine the order of magnitude of an exposure and to direct more detailed sampling to specific locations, task, or personnel.

The location of the sampling device is also important. A monitor placed at a static point in a workplace would collect an area sample. However, in many situations, that reading may not be fully representative of an individual's exposure to inhaled materials. For this reason, it is customary to obtain a breathing zone sample.

This is obtained by placing the collection device on a worker's lapel, helmet or other location to obtain samples that are representative of air inhaled by the person. This is particularly useful when people are around point sources of chemicals.

Passive dosimetry is a method of assessing external exposure to chemical agents. In this technique, a matrix is exposed to a contaminated environment and allowed to absorb agents of interest for later analysis.

Passive dosimeters are available for the determination of airborne exposure to chemicals and are easy to use. However, techniques involving passive samplers are generally considered less accurate than active sampling techniques, and active sampling methods are preferred when available.

Passive samplers do provide the benefit of being smaller and less cumbersome to use. They depend on the diffusion of the agent into the monitor and absorbing bed. They may be left in an area or on a person for a period of time and then sealed and analyzed at a remote laboratory.

This monitoring technique permits large-scale screening of workers and could be used to determine the likelihood of significant exposure. However, specific quantitative analyses may be more accurate with active sampling.

Instead of monitoring the general atmosphere of the workplace or the immediate environment of the worker, some hazards can be detected by biological monitoring. Biological monitoring has the intended advantage of measuring the chemical that has actually entered the body of the worker.

The sample may be sputum, urine, blood, fat, breath, hair, nails or other bodily materials. Exposure to metals can be detected this way.

People working with radioactive iodine should regularly have their thyroid glands in the neck monitored with a radioactive counter. This is because any iodine entering the body is concentrated in the thyroid and exposure can easily be measured.

There is also a direct co-relation between workers exposed to trichloroethylene, a general solvent used in metal cleaning processes, and the amount of trichloroacetic acid (TCA) found in their urine. But it must be remembered in all these examples that what we are measuring indirectly is how much of the chemical has already entered the body. In other words, it is too late.

Monitoring of health effects

In health effects monitoring, the organism is examined for known or suspected effects of the agent in question. Health effects monitoring may be graded by human epidemiological studies, case reports, animal tests and other sources of information about the likely target organ or other details of its effects on humans.

The important goal of health effects monitoring is to detect effects at the earliest possible stage to be able to intercede before significant or irreversible injury has taken place. Unfortunately, this is not always accomplished or possible.

Ideally, everybody using toxic chemicals should be afforded medical evaluation tailored to their exposure.

The writer is a Science Officer at the National Poison Centre, Universiti Sains Malaysia, Penang.

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