Safety in the Use of ChemicalsChapter 5Monitoring Exposure to Chemicals |
| Monitoring exposure to chemicals and their effects on the health of exposed persons is an essential part of health and safety measures. |
Once it is established that chemical hazards exist, it becomes necessary to evaluate them for their potential to cause harm to the safety and health of people, the surrounding communities and the ecology in general. Evaluation or risk assessment forms an essential component of any effort to combat occupational safety and health hazards.
Monitoring exposure and health forms an important component of risk assessment and helps define priorities for action to be taken to prevent damage to health and safety and avoid ecological and public health implications.
Personal monitoring
Basically, personal monitoring involves estimating or measuring the quantity of particular occupational health and safety hazards (in this case, the quantity of chemicals) to which the worker is exposed. It generally means measuring the chemicals in the air, as most exposure to chemicals occurs by this means. However, sometimes, we may be exposed through our skin; in such cases some other method has to be used to estimate exposure. The most important objectives of personal (or occupational) monitoring are these:
- Evaluation of actual and possible exposure to hazardous chemicals.
- Finding out data for compliance with national (or sometimes international) legislation and standards.
- Knowing whether control measures are working properly.
Since monitoring involves considerable expenditure of time and money, any monitoring strategy has to be devised very carefully. The strategy should be aimed at gaining maximum knowledge of exposure levels and their impact on the health and safety of people and general environment, within minimum possible time and expenditure of resources. Several factors have to be taken into consideration while carrying out monitoring of personal exposure. These may include:
- The physical and chemical properties of the contaminating chemical (whether liquid, gaseous, vapour or other form).
- The means of exposure.
- The inherent toxicity of the contaminating chemical: the more toxic the chemical, the more frequent and more detailed the sampling that has to be done.
- Some people may be at higher risk than the others because of their activities and require more attention.
- The patterns and intensity of exposure may vary during the work day or over a period of time. At certain times they may be at peak, while at others they may not be harmful.
Methods for personal monitoring
A brief discussion of the types of techniques used in personal monitoring will be helpful in making decisions for selecting a monitoring method. Some questions have to be asked before selection of the method. These are:
(a) How accurate and precise should the results be?
(b) Does the method meet national legislative requirements?
(c) Are backup services such as chemical analysis available locally?
(d) How much does it cost?
Direct reading instruments
One simple example of direct reading instruments is the ordinary thermometer. It gives an instantaneous reading of the temperature (of the body or the air). A large variety of instruments are now available which give instantaneous results of the amount of contamination present in the air. They may work by electronic, optical, chemical or by a combination of different principles.
The accuracy and consistency of instruments may vary from one technique to the other. Colour detection tubes are an easy tool for detection of the concentration of airborne contaminants and usually come in handy because of their ease of use and portability. These tubes are usually specific to a single chemical or a mixture of a class of chemicals (like hydrocarbons). A specific quantity of air is passed through the tube by manual or electrical pumps, the chemicals in the tube react with the contaminant and change their colour. The amount of contaminant is estimated then from the amount of colour change.
Other instruments give an indication of the level of contaminant either on a digital or an analogue dial. There are several varieties of such instruments. Some optical instruments measure particle size and the amount of airborne dust particles. Some instruments have chemical sensors, specific for chemicals like carbon monoxide, nitrous fumes, hydrogen sulphide, flammable gases, chlorine etc. Such instruments prove very useful in detecting leaks or in evaluating the situation before entering confined spaces or emergency sites.
There are some instruments like portable gas chromatograph and infrared analysers which can provide the data of air concentration in a number of chemicals in the air. Direct reading instruments are very useful in situations where a continuous record of air concentration or warning for leak or spill of hazardous chemicals is required.
Sampling pumps and accessories
These are often used for collecting the samples of the environment for later analysis using chemical or gravimetric techniques. These pumps are usually battery powered and are calibrated to suck air at a known rate through a capturing media (filters for aerosols, absorbent tubes for some organic vapours, chemical solutions for certain chemicals). The capturing media can be modified to collect only certain particle sizes or shapes of the airborne dusts. Some capturing media like diffusive samplers for organic vapours do not need a pump and collect the vapours on their own, which can be analysed later on like samples collected through the pumps.
Environmental monitoring
Emissions from factories and plants in the form of solid liquid or air wastes result in environmental pollution. Most countries have regulations and standards for controlling the amount of gaseous and liquid emissions from industries. A whole range of instrumentation and technologies exist for measuring the environmental pollution caused by industrial sources. The actual techniques depend again on the physical and chemicals form of the pollutants, the accuracy and precision required, the duration and frequency of sampling, and the legislative requirements from country to country.
Many of the analytical techniques used to assess personal exposure are also valid for environmental samples. Some of the portable meters for specific gases like sulphur dioxide used to assess exposure at the work site are also used for environmental monitoring. The different types of emissions typical of industries and the techniques for their monitoring are given in Table 5.1.
Table 5.1 Typical pollutants and monitoring/analytical techniques
| Type of waste | Pollutant to look for | Monitoring techniques |
| Smoke stack emissions | Oxides of carbon, nitrogen, sulphur, unburnt fuel, carbon particles, volatile organic chemicals, aerosols | Stack gas monitors of various types, using filtration, electrostatic, chemical detection, light scattering or chromatographic technologies |
| Liquid waste | pH
BOD (Biological oxygen Demand) COD (Chemical Oxygen Demand) Heavy metals Inorganic ions Temperature Turbidity Total Suspended Solids Pesticides |
pH metre or pH paper
BOD maters COD maters Special electrodes for certain metals, or liquid samples to be analyzed using various chemical techniques Special electrodes or chemical analysis of water Thermometer Analysis of water or turbidity meters Analysis of water sample Analysis of samples with techniques like High Performance Liquid Chromatography (HPLC) |
| Air pollution | Aerosols, trace metals and oxides,
oxides of nitrogen, sulphur, etc., nitrates, sulphates, acid content of air, volatile organic chemicals |
Fixed or mobile specialized sampling units of various types |
Medical monitoring
In many cases personal monitoring may not be sufficient. In such cases medical monitoring is advisable in order to assess the hazards. There are certain limitations of personal monitoring, which include:
- Personal monitoring is usually limited to air monitoring. Some people may be exposed through the skin; thus the actual exposure may be higher than indicated by the air sampling.
- The concentration of the contaminant may vary over time, and the air samples may not present the complete situation.
- Besides reinforcing the findings from personal monitoring, medical monitoring serves several useful purposes in risk assessment and management in industry.
- A disease still not evident from symptoms may be detected if proper examination is carried out at regular intervals.
- A pre-employment medical check-up may indicate the individuals with susceptibility to allergic and other reactions to the contaminants likely to be encountered in the workplace.
- Many types of occupations will be examined under statutory requirements or company policies.
- The pre-employment and post-employment medical records are important for settling claims for compensation, etc.
The exact nature and frequency of medical monitoring will be determined by such factors as the hazards in specific occupations, company and statutory requirements. Some of the specific tests important for assessing exposure to chemicals include lung function testing, urine and blood level of hazardous materials or their metabolites, level of contaminants or metabolites in the exhaled air, and other clinical diagnostic tests used routinely by doctors .
Exposure limits
To interpret the results of monitoring, there must be some kind of reference to compare with. Many countries have such references in their laws, called exposure limits. These limits usually express the quantity of the hazard (chemicals in air or body or physical hazards in the environment) to which a person can be exposed without ill health effects. These limits may be applicable to the total exposure during a full working shift (known as TWA or time weighted average).
In the case of certain chemicals, short-term exposure limits (STELs), peak values and ceiling limits are also prescribed. In many cases an action level is also given. It is lower than the exposure limit and serves as a warning that some kind of action should be taken before the hazard can touch the actual exposure limit. Biological limits for the allowable levels of harmful substances in blood, urine or other body parts have also been specified for certain chemicals. These limits are very helpful in deciding if a particular chemical is a hazard to the health and safety of the exposed persons.
Exercise
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