When toxic chemicals are present in the workplace, employee exposure can be estimated by measuring the concentration of a given chemical in the air and the duration of exposure. This measurement is called air or environmental monitoring or sampling and is usually done by industrial hygienists, using various types of instruments. The air is collected from the employee's breathing zone (the air around the nose and mouth) so that the concentrations measured will accurately reflect the concentration that is inhaled. The exposure levels calculated from this monitoring can then be compared to the Permissible Exposure Level for that chemical.

Environmental monitoring is the most accurate way to determine a person's exposure to most chemicals. However, for chemicals that are absorbed by routes other than inhalation, such as through the skin and by ingestion, air monitoring may underestimate the amount of chemical that is absorbed by an employee. For these and some other chemicals, the levels of the chemical (or its breakdown products) in the body can sometimes be measured in the blood, urine or exhaled air. Such testing is called biological monitoring, and the results may give an estimate of the actual dose absorbed into the body. For one substance, lead, biological monitoring is required by law when air monitoring results are above a certain level. The American Conference of Governmental Industrial Hygienists (ACGIH) has recommended the exposure limits for biological monitoring for a small number of chemicals. These are called Biological Exposure Indices (BEIs) and are published together with TLVs.

Odor: Some chemicals can be smelled at levels well below those that are harmful, so that detecting an odor does not mean that you are inhaling harmful amounts. On the other hand, some chemicals can be present without generating an odor. Some chemicals cannot be smelled even at levels that are harmful.

The odor threshold is the lowest level of a chemical that can be smelled by most people. If a chemical's odor threshold is lower than the amount that is hazardous, the chemical is said to have good warning properties. One example is ammonia. Most people can smell it at 5 ppm, below the PEL of 25 ppm. It is important to remember that for most chemicals, the odor thresholds vary widely from person to person. In addition, some chemicals, like hydrogen sulfide, cause you to rapidly lose your ability to smell them (called olfactory fatigue). With these cautions in mind, knowing a chemical's odor threshold may serve as rough guide to your exposure level.

Don't depend on odor to warn you. Remember that your sense of smell may be better or worse than average, that some very hazardous chemicals have no odor (carbon monoxide), some chemicals of low toxicity have very strong odors (mercaptans added to natural gas), and others produce olfactory fatigue.

Taste: If you inhale or ingest a chemical, it may leave a taste in your mouth. Some chemicals have a particular taste, which may be mentioned in an MSDS.

If you cough up mucous with particles in it, or blow your nose and see particles on your handkerchief, then you have inhaled some chemical in particle form. Unfortunately, most particles which are inhaled into the lungs are too small to see.

If chemical dust or mist is in the air, it will eventually settle on work surfaces or on your skin, hair and clothing. It is likely that you inhaled some of this chemical while it was in the air.

If you or your co-workers experience symptoms known to be caused by a chemical during or shortly after its use, you may have been overexposed. Symptoms might include tears in your eyes; a burning sensation of skin, nose, or throat; a cough; dizziness or a headache.

Medical surveillance is a program of medical examinations and tests designed to detect early warning signs of harmful exposure. A medical surveillance program may discover small changes in health before severe damage occurs. Testing for health effects is called medical monitoring. The type of testing needed in a surveillance program depends upon the particular chemical involved. Unfortunately, medical monitoring tests that accurately measure early health effects are available only for a small number of chemicals. A complete occupational surveillance program should consist of industrial hygiene monitoring, medical monitoring, and biological monitoring when appropriate. Tests for health effects when you are already sick are not part of medical surveillance, and must be selected by your physician on a case by case basis.

There are short-and long term sampling devices for gases and vapors. Sampling methods can be classified into one of three categories:

1. Full-period, Continuous Single Sampling. Full-period, continuous single sampling is defined as sampling over the entire sample period with only one sample. The sampling may be for a full-shift sample or for a short period ceiling determination.

2. Full-period, Consecutive Sampling. Full-period, consecutive sampling is defined as sampling using multiple consecutive samples of equal or unequal time duration which, if combined, equal the total duration of the sample period. An example would be taking four 2-hour charcoal tube samples. There are several advantages to this type of sampling:

3. Grab Sampling. Grab sampling is defined as collecting a number of short-term samples at various times during the sample period which, when combined, provide an estimate of exposure over the total period. Common examples include the use of detector tubes or direct-reading instrumentation (with intermittent readings).

Short term samples involve drawing air into an evacuated, contaminant free container or pumping a sample through an impregnated filter paper or gas detection tube. Detector tubes and pumps are screening instruments which may be used to measure more than 200 organic and inorganic gases and vapors or for leak detection. Some aerosols can also be measured.

A limitation of many detector tubes is the lack of specificity. Many indicators are not highly selective and can cross-react with other compounds. Manufacturers' manuals describe the effects of interfering contaminants.

Another important consideration is sampling time. Detector tubes give only an instantaneous interpretation of environmental hazards. This may be beneficial in potentially dangerous situations or when ceiling exposure determinations are sufficient. When long-term assessment of occupational environments is necessary, short-term detector-tube measurements may not reflect time-weighted average levels of the hazardous substances present.

Continuous or long-term samples require more elaborate equipment. Sample times are typically 15 to 30 minutes. Some systems continuously sample many ports or sensors to detect if a leak or release of hazardous material occurs. Portable, powered air sampling pumps draw air through a variety of collection devices. They are calibrated so that the volume of air drawn through the collection device is known precisely. Some collectors contain an adsorbent material (Charcoal or silica gel) that accumulates the substance. After collection, samples are analyzed in a variety of ways to assess the type and concentration of contaminants.

There are many ways to identify a type of contaminant and to assess concentration. The proper method depends on the form of the material, type of material and collection method used.

Colorimetric Methods: This method causes a reactive material to produce a color in proportion to the quantity of a substance that is present. Figure 1 shows a colorimetric kit which uses a color comparator. The results can also be analyzed by a digital photometer as shown in Figure 2 .

Gravimetric Method: In this method the amount of a substance is measured by weighing either the collected substance or the product of a reaction.

Volumetric Method: This method involves reaction with definite volumes of standard solutions or reagents.

Ion Exchange Chromatography: Ion exchange chromatography is commonly used in the purification of biological materials. There are two types of exchange: cation exchange in which the stationary phase carries a negative charge, and anion exchange in which the stationary phase carries a positive charge. Charged molecules in the liquid phase pass through the column until a binding site in the stationary phase appears. The molecule will not elute from the column until a solution of varying pH or ionic strength is passed through it.

Gas Chromatography: Gas chromatography makes use of a pressurized gas cylinder and a carrier gas, such as helium, to carry the solute through a coiled column of glass fused silica oxide-glass located inside an oven. The most common detectors used in this type of chromatography are thermal conductivity and flame ionization detectors. The sample in the column is heated until the components vaporize. Because a specific type of molecule possesses a characteristic size and boiling point, each chemical compound is carried through the column at different times. The period between the injection of a compound and its detection by a sensor is called retention time (RT). Retention time is a specific behavior for every compound. Therefore, comparing sample RT with standard RT, the compounds are identified and quantified.

There are three types of gas chromatography: gas adsorption, gas-liquid and capillary gas chromatography.

Gas adsorption chromatography involves a packed bed comprised of an adsorbent used as the stationary phase. Common adsorbents are zeolite, silica gel and activated alumina. This method is commonly used to separate mixtures of gases.

Gas-liquid chromatography is a more common type of analytical gas chromatography. In this type of column, an inert porous solid is coated with a viscous liquid which acts as the stationary phase. Diatomaceous earth is the most common solid used. Solutes in the feed stream dissolve into the liquid phase and eventually vaporize. The separation is thus based on relative volatilities.

Capillary gas chromatography is the most common analytical method. Glass or fused silica comprise the capillary walls which are coated with an absorbent or other solvent. Because of the small amount of stationary phase, the column can contain only a limited capacity. However, this method also yields rapid separation of mixtures

X-ray Fluorescence Spectrometry: This technique is used to determine the major and trace elements in the chemical composition of such materials as ceramics, and glass. A sample is bombarded with X-rays, and the wavelengths of the released energy, or fluorescent X-rays, are detected and measured. Different elements have unique wavelengths, and their concentrations can be estimated from the intensity of the released X-rays. This analysis may, for example, help an archaeologist in identifying the source of the material.

X-ray Diffraction: This method is used to study the atomic and molecular structure of crystalline substances by using X-rays. X-rays directed at such substances spread out as they pass through the crystals owing to diffraction (the slight spreading of waves around the edge of an opaque object) of the rays around the atoms. By using measurements of the position and intensity of the diffracted waves, it is possible to calculate the shape and size of the atoms in the crystal. The method has been used to study substances such as DNA that are found in living material.

Spectroscopy : This method involves affecting a sample with carbon arc, infrared radiation, electron beams or high temperature flame. Each process produces a spectrum that gives a unique signature for particular materials.

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Last Update: March 4, 2000

By: Serdar Z. Elgun