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Health effects

Terminology of the health effects of elements and compounds

CAS numbers - IMAC - MAC - PEL - Ceiling values - TWA - STEL - TLV

CAS numbers

CAS (Chemical Abstract Service) numbers represent chemical substances recorded in the CAS Chemical Registry System. This numbering system identifies chemical substances by an unambiguous computer language description of its molecular structure, including all stereo-chemical detail. The CAS number, which has no chemical significance, is simply a number assigned in sequential order to each substance as it enters the Registry System. All specific substances reported in the world's scientific and technical literature and indexed in Chemical Abstracts (CA) since 1965 (when the Registry System began) are included in the master file.


Interim Maximum Acceptable Concentration. This is a health-related Ontario drinking water standard established for contaminants when there are insufficient toxicological data to establish a MAC with reasonable certainty, or when it is not practical to establish a MAC at the desired level.


Maximum Acceptable Concentration. This is a health-related Ontario drinking water standard established for contaminants that have known or suspected adverse health effects when above a certain concentration. The length of time the MAC can be exceeded without injury to health will depend on the nature and concentration of the parameter.


Permissible Exposure Limit. It’s the maximum amount or concentration of a chemical that a worker may be exposed to under OSHA (Ocupational Health and Safety Administration) regulations.

PEL's can be defined in two different ways as discussed in the OSHA regulation on air contaminants, 1910.1000:

· Ceiling values - at no time should this exposure limit be exceeded. Sometimes denoted with the letter C.

· 8-hour Time Weighted Averages (TWA) - are an average value of exposure over the course of an 8 hour work shift.
TWA levels are usually lower than ceiling values. Thus, a worker may be exposed to a level higher than the TWA for part of the day (but still lower than the ceiling value) as long as he is exposed to levels below the TWA for the rest of the day.


Short Term Exposure Limit. It’s defined by the ACGIH (American Conference of Governmental Industrial Hygienists) as the concentration to which workers can be exposed continuously for a short period of time without suffering from:

· Irritation

· Chronic or irreversible tissue damage

· Narcosis of sufficient degree to increase the likelihood of accidental injury, impair self-rescue or materially reduce work efficiency.

STEL’s are generally used only when toxic effects have been reported from high acute (short-term) exposures in either humans or animals. An STEL is not a separate independent exposure limit, but supplements time-weighted average limits where there are recognized acute effects from a substance whose toxic effects generally chronic (long-term) in nature.

For example, one cannot be exposed to an STEL concentration if the TLV-TWA (time weighted average for an 8 hour shift; see Permissible Exposure Limit (PEL)) would be exceeded. Workers can be exposed to a maximum of four STEL periods per 8-hour shift, with at least 60 minutes between exposure periods.


Threshold Limit Values are guidelines (not standards) prepared by the American Conference of Governmental industrial Hygienists, Inc (ACGIH) to assist industrial hygienists in making decisions regarding safe levels of exposure to various hazards found in the workplace.

A TLV® reflects the level of exposure that the typical worker can experience without an unreasonable risk of disease or injury. TLVs® are not quantitative estimates of risk at different exposure levels or by different routes of exposure.

Some definitions were taken from The MSDS Hyperglossary (http://www.ilpi.com/msds/ref/).

To choose the health effects per chemical element go to the periodic table of elements

and here to see the definitions and explanations of the effects of chemical elements in the environment.

also available: The health effects regarding radiation

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