Airborne contaminant exposure standard

Exposure standards represent airborne concentrations of individual chemical substances which, according to current knowledge, should neither impair the health of nor cause undue discomfort to nearly all workers. Additionally, the exposure standards are believed to guard against narcosis or irritation that could precipitate occupational accidents.

Threshold limit value (TLV) refers to airborne concentrations of substances to which it is believed that nearly all workers may be repeatedly exposed day after day without adverse effect.

There are three categories of exposure standards:

• 8-hour time-weighted averages (TWAs)
• Short-Term Exposure Limits (STELs)
• Peak Limitations or Ceiling Values

8-hour time-weighted averages (twas): average airborne concentration of a particular substance when calculated over a normal eight-hour working day, for a five-day working week.

8-Hour TWA exposures are calculated as follows:

TWA = (C₁T₁ + C₂T₂ + C₃T₃... + C_NT_N) /8

where:

C = concentration of contaminant; and

T = incremental exposure time.

Short term exposure limits (stels): Exposures at the STEL should not be longer than 15 minutes and should not be repeated more than four times per day. There should be at least 60 minutes between successive exposures at the STEL It is to avoid both acute and chronic health effects.

Some substances can cause intolerable irritation or other acute effects upon brief overexposure, although the primary toxic effects may be due to long-term exposure through accumulation of substances in the body or through gradual health impairment with repeated exposures. Under these circumstances, exposure should be controlled not to exceed STEL to avoid both acute and chronic health effects.

Peak limitations are concentrations that should not be exceeded even for an instant during any part of the workday.

For some rapidly acting gases or vapours, the averaging of the airborne concentration over an eight-hour period is inappropriate.

These substances may induce acute effects after relatively brief exposure to high concentrations and so the exposure standard for these substances represents a maximum or peak concentration to which workers may be exposed. Examples of gases or vapours with peak limitation exposure standards are hydrogen fluoride, acetic anhydride, n-butyl alcohol, chlorine, ethyl acrylate, ozone and glutaraldehyde.

Airborne contaminants are classified into 4 classes by their TWA:

1^st class – extremely hazardous substances with TWA less 0.1 mg/m³ (lead, mercury, ozone);

2^nd class – highly hazardous substances with TWA within 0.1 - 1.0 mg/m³ (sulfuric and hydrochloric acid, chlorine, phenol);

3^rd class – medium hazardous substances with TWA within 1.1 - 10.0 mg/m³ (toluene, methyl spirit);

4^th class – low hazardous substances with TWA greater 10 mg/m³ (ammonia, gasoline, acetone).

Standard may have a letter that points effect to be produced by contaminant being exposed to the person, for example “O” – acute effect; “A” – allergic effect; “K” – carcinogenic effect; “F”- fibrotic effect.

If there’re some multidirectional hazardous substances simultaneously being in the air of the work zone then concentration of everyone shouldn’t exceed correspondent LAC.

Airborne contaminants may force to have either antagonistic or synergistic effects.

In first case they are of multidirectional action, in the second – unidirectional.

Acceptable condition for multidirectional hazards C1£TWA1, C2£TWA2...

Acceptable condition for unidirectional ones

Variety of sampling methods are used in sampling of gases, vapors and dust:

- express method, based on colorimetry method;

- laboratory method, that implies sampling in work zone with further physical and chemical analysis in laboratory conditions;

- automatic control method using gas analyzers and gas signaling systems.

Class of substance, concentration and hazard of health impact define periodicity of sampling.

To secure workplace atmospheres, which are as free as practicable from hazardous contaminant following control, measures are developed:

- substituting a dangerous substance with one that is not as dangerous;

- improving technology;

- remote control of manufacturing process;

- isolation of manufacturing equipment, using local ventilation;

- normal operation of heating system;

- medical examination of workers working with harmful substances;

- airborne contaminants sampling in the air of work zone;

- using personal protective equipment.