Use Of Other Approaches

When no RfD has been derived, regulators can use the level at which no observed adverse effects have been found (NOAEL) or the lowest level at which adverse effects have been observed (LOAEL), with appropriate safety factors. These levels are similar in nature and use to the RfDs. Related levels are the no observed effect level (NOEL) and the lowest observed effect level (LOEL), respectively. Other sources of information are the minimal risk level (MRL), the level that is immediately dangerous to life and health (IDLH), emergency response planning guidelines (ERPG), and emergency exposure guideline levels (EEGL) for specific pollutants. These last four levels are for special situations; for these levels to be useful in assessing danger to the general public, regulators must severely attenuate them by safety factors. However, in the absence of other data, these levels can be useful in establishing an ACL or standard.

A pollutant's NOAEL is the highest tested experimental exposure level at which no adverse effects are observed. The NOEL is the highest exposure level at which no ef fects, adverse or other, are observed. The NOEL is generally less useful since factors other than toxicity can produce effects.

A pollutant's LOAEL is the lowest tested experimental exposure level at which an adverse health effect is observed. Since the LOAEL does not convey information on the no-effect level, it is less useful than the NOAEL, but it can still be useful. The LOEL is the lowest level at which any effect is observed, adverse or not. As a result, it is generally less useful than the NOEL.

MRLs are derived by the Agency for the Toxic Substances and Disease Registry (ATSDR), which was formed under the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) of 1980. The CERCLA requires ATSDR to prepare and update toxicological profiles for the hazardous substances commonly found at superfund sites (those sites on the National Priority List) that pose the greatest potential risk to human health. As part of the profiles, ATSDR derives MRLs for both inhalation and ingestion exposures.

The National Institute for Occupational Safety and Health (NIOSH) developed IDLHs primarily to select the most effective respirators to use in the work place. IDLHs are the maximum pollutant concentration in the air from which healthy male workers can escape without loss of life or suffering irreversible health effects during a maximum thirty-minute exposure. Another way of thinking of IDLHs is that if levels are above these standards, respirators must be used to escape the area of contamination.

The American Industrial Hygiene Association (AIHA) has derived ERPGs at three levels for several substances. Level 1 is the lowest level; it represents the maximum pollutant concentration in the air at which exposure for one hour results in mild, transient, adverse health effects. Level 2 is the concentration below which one hour of exposure does not result in irreversible or serious health effects or


Pollutant Averaging Time Primary Secondary

Particulate matter, 10 micrometers (PM10) Sulfur dioxide (SO2)

Carbon monoxide (CO) Ozone (O3)

Nitrogen dioxide (NO2) Lead (Pb)

Annual arithmetic mean 24-hour

Annual arithmetic mean





1-hour per day Annual arithmetic mean Quarterly arithmetic

50 Mg/m3 150 Mg/m3

9 ppm (10 mg/m3) 35 ppm (40 mg/m3) 0.12 ppm (235 Mg/m3) 0.053 ppm (100 Mg/m3) 1.5 Mg/m3

Same as Same as Same as Same as 0.5 ppm Same as Same as Same as Same as Same as primary primary primary primary

primary primary primary primary primary mean

Source: CFR Title 40, Part 50. Environmental Protection Agency. U.S. Government Printing Office, 1993.

Notes: All standards with averaging times of 24 hours or less, and all gaseous fluoride standards, are not to have more than one actual or expected exceedance per year.

^g/m3 or mg/m3 = microgram or milligram per cubic meter

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