8. Measuring airborne concentrations and air protection

Personal exposures are measured in the breathing zone of the worker.

Breathing zone: is described by a hemisphere of 300 mm radius extending in front of a worker's face and measured from the midpoint of an imaginary line joining the ears.

There are two general types of sampling techniques:

• active Sampling: using pump;

• passive Sampling: no pump.

There are three key elements of active sampling:

• a sampling pump - something to pull or push air;

• the sampling media - something to pull or push air through;

• a calibrator - something to indicate how much air has been pulled or pushed.

A variety of sampling media is used in sampling for gases and vapours:

• sorbent tubes;

• filters;

• impingers: are specially designed glass bottles that are filled with a collection liquid specified in the sampling method for specific chemicals;

• sampling bags;

• passive samplers: the collection of airborne gases and vapours at a rate controlled by a physical process such as diffusion through a static air layer or permeation through a membrane WITHOUT the active movement of air through an air sampling pump.

Requirements for sampling instruments are:

• to detect airborne contaminant at least at concentration a half of exposure standard in exhaust air;

• and the third of exposure standard in influx air;

• acceptable error is 25% of measured value.

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

In first case they are of multidirectional action, in the second – unidirectional. Those types should be considered for assessing concentrations against the standards.

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

Acceptable condition for unidirectional ones

Atmospheric air can be referred to inexhaustible resources. However air protection takes an important place in ecology and offers variety of purification methods.

Purification from dust: dust catchers «dry» and scrubbers «wet».

Purification from gases: absorption methods, adsorption methods.

Absorption methods remove airborne contaminants by the liquid.

In absorber gas and liquid move contraflow what’s followed by chemical reaction.

Absorbers advantages: simple, reliable and have high degree of purification.

Disadvantages: cumbersome equipment, salvaging wastes.

Adsorption methods trap airborne chemicals on the surface of solid bodies (sorbents).

Adsorbers advantages: no chemical reaction, adsorbed gases and sorbent can be recycled.

Practice:

Assess the given concentrations of airborne contaminants: 10 mg/m³ of white spirit and 0.01 mg/m³ of lead, which make a group of unidirectional contaminants, and; 50 mg/m³ of gasoline.

9. 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.