Normal ambient air contains an oxygen concentration of 20.8% by volume. When the oxygen level dips below 19.5%, the air is considered oxygen deficient.

Oxygen concentrations below 16% are

considered unsafe for human exposure because

of harmful effects on bodily functions, mental

processes, and coordination. It is important to note that life-supporting oxygen can be further displaced by other gases, such as carbon dioxide. When this occurs, the result is often an atmosphere that can be dangerous or fatal when inhaled. Oxygen deficiency also can be

caused by rust, corrosion, fermentation, or other forms of oxidation which consume oxygen. The impact of oxygen-deficiency can be gradual or sudden.

Particulate contaminants can be classified

according to their physical and chemical

characteristics and their physiological effect on

the body. The particle diameter in microns

(1 micron = 1/25,400 inch) is of utmost

importance. Particulates below 10 microns in

diameter have a greater chance to enter the

respiratory system, and particles below 5 microns in diameter are more apt to reach the deep lung or alveolar spaces.

In healthy lungs, particles from 5 to 10 microns

in diameter are generally removed from the

respiratory system by a constant cleansing action that takes place in the upper respiratory tract.

However, with excessive "dust" exposures or a

diseased respiratory system, the efficiency of the cleansing action can be significantly reduced.

The various types of airborne particulate

contaminants can be classified as follows:

produced solid particles derived from the breaking up of larger particles. Dusts generally have a larger particle size when compared to fumes.

Operations such as sanding, grinding, crushing, drilling, machining, or sand blasting are the worst

dust producers. Dust particles are often found in the harmful size range of 0.5 to 10 microns.

Gas and vapor contaminants can be classified

according to their chemical characteristics.

True gaseous contaminants are similar to air in

that they possess the same ability to diffuse

chlorine, carbon monoxide, carbon dioxide, and

sulfur dioxide are examples.

In terms of chemical characteristics, gaseous

contaminants may be classified as follows:

helium, argon, neon, etc. Although they do not

metabolize in the body, these gases represent a

hazard, because they can produce an oxygen

deficiency by displacement of air.

exist as acids or produce acids by reaction with

water. Sulfur dioxide, hydrogen sulfide and

hydrogen chloride are examples.

Vapors are the gaseous state of substances that are liquids or solids at room temperature. They are formed when the solid or liquid evaporates.

In terms of chemical characteristics, vaporous

contaminants may be classified as follows:

category can exist as true gases or vapors

produced from organic liquids. Gasoline,

solvents, and paint thinners are examples.

generally comprised of metals attached to

organic groups. Tetraethyllead and organic

phosphates are examples.

Proper assessment of your specific hazard (s) is the first important step to protection. This requires a thorough knowledge of processes, equipment, raw materials, end-products, and by-products that can create an exposure hazard. First, you must make an initial determination of workplace conditions.

This simple calculation of exposures does

not require sampling of the environment. Factor in workplace size, ventilation, the amount of the regulated substance present, the type of operation, and the proximity of the workers to the source of emissions.

According to OSHA, personal exposure

monitoring is the "gold standard" for determining employee exposure. It is the most reliable approach for assessing the level and type of respiratory protection required. Sampling which uses methods appropriate for contaminants (s) should represent the worst-case exposures or enough shifts and operations to determine the accurate range (s) of exposure.

To determine an atmosphere’s oxygen content

or concentration levels of particulate and/or

gaseous contaminants, air samples must be taken with proper sampling instruments during all conditions of operation. The sampling device and the type and frequency of sampling (spot testing or continuous monitoring) will be dictated by the exposure and operating conditions.

Breathing zone samples are recommended and

sampling frequency should be sufficient to assess the average exposure under the variable operating and exposure conditions.

Should contaminant concentrations exceed

exposure limits recommended by the American

Conference of Governmental Industrial Hygienists

(ACGIH), OSHA, or NIOSH, hazard control

procedures must be implemented promptly.

OSHA emphasizes that, because workplaces

differ, each respirator program must be tailored

to the specific conditions of the workplace.

The recently revised OSHA standard serves as a "building block standard", because it

consolidates respiratory protection provisions

that previously were included in sections dealing with specific chemical substances.

The employer is required to evaluate respiratory hazards in the workplace, identify relevant workplace and use factors, and base respirator selection on these factors.

Exposure monitoring plays a critical role in the

respirator selection process. The results from

such testing will help you determine whether

respiratory protection is needed and, if it is,

the type of respirator that is required. Generally,

respirator selection is based on three factors:

• The results of your atmospheric monitoring or sampling program

• The accepted ACGIH, OSHA, or NIOSH

exposure limits for the substance (s) present

• The maximum use concentration (of a

substance) for which a respirator can be used

Exposure limits include:

ACGIH Threshold Limit Values (TLVs)

OSHA Permissible Exposure Limits (PELs)

NIOSH-Recommended Exposure Levels (RELs) AIHA Workplace Environmental Exposure Levels (WEELs) These values are guides for exposure concentrations that healthy individuals can normally tolerate for eight hours a day, five days a week, without harmful effects. Unless otherwise noted, exposure limits are eight-hour,

time-weighted-average (TWA) concentrations.

In general, gas and vapor exposure limits

are expressed in ppm by volume (parts of