Exposure standards

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:

, (2)

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 OR CEILING VALUES: concentrations that should not be exceeded even for an instant during any part of the workday.

For some rapidly acting gases or vapors, 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. 1^st class – extremely hazardous substances with TWA less 0.1 mg/m³ (lead, mercury, ozone);

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

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

4. 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.

Дата добавления: 2014-01-04