Graph 4  Example of an Exposure Compared to an STEL and a TWA

(Created 12/02) 
UNL Environmental Health and Safety · (402) 472-4925 · http://ehs.unl.edu 
appropriate. ACGIH and OSHA state that a ceiling value should not be exceeded even 
instantaneously. . They denote a ceiling valuely a ACT preceding the exposure limit.
NIOSH also uses ceiling values. However, their ceiling values are more like a STEL. Many have 
time limits (from 5 to 60 minutes) associated with the exposure. Graph 5 illustrates an exposure 
that does not exceed a ceiling value of 5 ppm.
Graph 5 
Example of an Exposure Compared to a Ceiling Exposure Limit 
Peaks. Until recently ANSI, and OSHA where they have adopted ANSI standards, had used a 
peak exposure limit. This peak exposure is an allowable excursion above their ceiling values. 
The duration and number of exposures at this peak value is limited. For example, ANSI allowed 
the 25 ppm ceiling value for benzene to be exceed to 50 ppm but only for 10 minutes during an 8 
hour period. ANSI withdrew its exposure limit standards in 1982. With the revision of the PELs 
in 1989, OSHA has dropped most of its peak values.
"Skin" Notation. While these exposure guidelines are based on exposure to airborne 
concentrations of chemicals. However, OSHA, NIOSH, ACGIH and AIHA recognize that there 
are other routes of exposure in the workplace. In particular, there can be a contribution to the 
overall exposure from skin contact with chemicals that can be absorbed through the skin. 
Unfortunately, there is very little data available that quantifies the amount of allowable skin 
contact. But some organizations provide qualitative information about skin absorbable 

(Created 12/02) 
UNL Environmental Health and Safety · (402) 472-4925 · http://ehs.unl.edu 
chemicals. When a chemical has the potential to contribute to the overall exposure by direct 
contact with the skin, mucous membranes or eyes, it is given a "skin" notation.
This "skin" notation not only points out chemicals that are readily absorbed through the skin, but 
also notes that if there is skin contact, the exposure guideline for inhalation may not provide 
adequate protection. The inhalation exposure guidelines are designed for exposures only from 
inhalation. If additional routes of exposure are added, there can be detrimental effects even if the 
exposure guideline is not exceeded.

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