People have long recognized that no substance is absolutely safe. The Swiss physician Paracelsus, who lived from 1493 to 1541, said:
All substances are poisons; there is none which is not a poison. The right dose differentiates a poison and a remedy.
Knowing the “right dose” is critical to health. When talking about airborne materials the term “dose” includes two components: the toxicity of a substance (which is related to concentration) and the duration of exposure. The longer the exposure, the lower the concentration must be in order for people to be safe. OSHA has defined several exposure limits. The Permissible Exposure Limit (PEL) is the limit on what a person can be exposed to in an eight-hour day. The Short Term Exposure Limit (STEL) is the acceptable exposure limit over a short-term, usually 15 minutes.
Short Term Exposure Limit – Types of Hazards
Airborne hazards in the workplace can have serious effects on people. When evaluating chemical exposures the following types of effects are considered by OSHA:
- Neuropathic Effects
- Narcotic Effects
- Sensory Irritation
- Liver or Kidney Effects
- Ocular Effect (Eyes or Vision)
- Respiratory Effects
- Cardiovascular Effects
- Systemic Toxicity
- Physical Irritation
- Odor Effect
- Biochemical and Metabolic Effects
- Sensitization Effects
In addition, short term exposure limits are established for exposure to other types of hazards that have negative health effects, such as high levels of sound.
Short Term Exposure Limits – Definition
What is a Short Term Exposure Limit? In the book “Guidelines for Technical Planning for On-Site Emergencies” the Center for Chemical Process Safety defines a short term exposure limit as:
The concentration to which workers can be exposed continuously for a short period of time without suffering from:
2. chronic or irreversible tissue damage
3. narcosis of sufficient degree to increase the likelihood of accidental injury.
Provided the daily TWA is not exceeded.
The term “TWA” is the Time Weighted Average of exposure over an eight hour period. OSHA's 1910.1000(b)(1) standard states:
8-hour time weighted averages. An employee's exposure to any substance listed in Table Z-2, in any 8-hour work shift of a 40-hour work week, shall not exceed the 8-hour time weighted average limit given for that substance in Table Z-2.
OSHA does not commonly use the term “short term exposure limit,” but instead has established Permissible Exposure Limits (PEL), ceiling limits, and peak limits. The OSHA ceiling limit is similar to a short term exposure limit and is defined in 1910.1000(a)(1) as:
Substances with limits preceded by "C" - Ceiling Values. An employee's exposure to any substance in Table Z-1, the exposure limit of which is preceded by a "C", shall at no time exceed the exposure limit given for that substance. If instantaneous monitoring is not feasible, then the ceiling shall be assessed as a 15-minute time weighted average exposure which shall not be exceeded at any time during the working day.
Short Term Exposure Limits – Measuring Exposure
Most substances do not have a short term exposure limit. Only when a substance has a toxic effect, usually from a high concentration over a short time period, will there be an established STEL. In most cases the PEL (TWA) is sufficient to protect health. What this means is that the short term limit would need to be set at such a high level that the PEL would be exceeded. So there is no need for a short term exposure limit.
The measurement of air contaminants depends on the type of contaminant. For a few types of contaminants, such as combustible gases, there are direct measurement methods. However, in most cases air samples must be collected and analyzed in a laboratory. Various types of sample collection systems are available. The right one needs to be selected based on the type of contamination being measured.
In 1910.1000(d) OSHA gives the following formula to be used in calculating the TWA:
Computation formula. The computation formula which shall apply to employee exposure to more than one substance for which 8-hour time weighted averages are listed in subpart Z of 29 CFR Part 1910 in order to determine whether an employee is exposed over the regulatory limit is as follows:
The cumulative exposure for an 8-hour work shift shall be computed as follows:
E = (Ca Ta+Cb Tb+. . .Cn Tn)÷8
E is the equivalent exposure for the working shift.
C is the concentration during any period of time T where the concentration remains constant.
T is the duration in hours of the exposure at the concentration C.
The short term exposure could be measured in a similar manner or in most cases simply by collecting a sample over a 15 minute period.
Short Term Exposure Limit – Posting Warnings
After all engineering controls to eliminate the hazard have been used, should there be a situation in which someone may be exposed to a hazard that exceeds the short term exposure limit, warning signs and labels should be used. If administrative controls are being used, than employees must not enter certain areas. Those areas (including doorways and other access points) should be marked with either danger or warning signs. If some employees are allowed in areas in which the short term exposure limit may be exceeded, signs should inform them about the hazard and provide information about the required PPE.
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