Pennsylvania Code › Title 25 - ENVIRONMENTAL PROTECTION › Part I - Department of Environmental Protection › Subpart A - Preliminary Provisions › Article II - Statements of Policy › Chapter 16 - WATER QUALITY TOXICS MANAGEMENT STRATEGY-STATEMENT OF POLICY › Subchapter A - GUIDELINES FOR DEVELOPMENT OF CRITERIA FOR TOXIC SUBSTANCES AND WATER QUALITY CRITERIA FOR TOXIC SUBSTANCES › GUIDELINES FOR DEVELOPMENT OF HUMAN HEALTH-BASED CRITERIA › Section 16.32 - Threshold level toxic effects

Current through Register Vol. 54, No. 28, July 13, 2024
(a) A
						  threshold effect is defined as an adverse impact that occurs in the exposed
						  individual only after a physiological reserve is depleted. For these effects
						  there exists a dose below which no adverse response will occur. Threshold toxic
						  effects include most systemic effects and developmental toxicity, including
						  teratogenicity. Developmental toxicity includes all adverse effects in
						  developing offspring resulting from prenatal exposure to a causative
						  agent.
(b) Control of threshold
						  toxics is based upon animal testing or epidemiological studies that report no-
						  or lowest-observed adverse effect levels of the substance (NOAEL or LOAEL). In
						  evaluating a particular toxic, toxicologists weigh the merits of all the tests,
						  and choose, in their best professional judgment, the safe level. By applying
						  standard margins of safety to the NOAEL, extrapolations from the laboratory
						  animals to humans (factor of 10), for sensitive subpopulations (10), and from
						  short-term to chronic studies (10) can be taken into account. An additional
						  factor of 10 is used if only a LOAEL is available. Modifying factors (1-10),
						  which account for deficiencies in the toxicity studies, are also considered in
						  determining an acceptable exposure level. The current term for this acceptable
						  level is reference dose (RfD); it was previously called the acceptable daily
						  intake (ADI). Adverse effect levels may be calculated using Benchmark Dose
						  (BMD) Modeling. The purpose of the BMD is to derive a point of departure for
						  calculating a risk value, such as a reference dose or a reference
						  concentration. In the customary approach, the point of departure is the NOAEL
						  or the LOAEL. The BMD values are calculated by dividing a point of departure by
						  the uncertainty factors. This most sensitive effect is also called the critical
						  effect, and it is used as the point of departure in establishing a toxicity
						  benchmark. The RfD, can be calculated using a LOAEL, a NOAEL or BMD. It is
						  adjusted for protection of an average (80 Kg) person. It is then divided by
						  expected exposure conditions to result in an applicable criterion. Exposure
						  conditions by means of water include 2.4 liters per day of drinking water and
						  consumption of 22.0 grams of fish per day. The bioaccumulation of toxics in
						  edible portions of fish is accounted for by use of bioaccumulation factors
						  (BAF). The BAF is the ratio in liters per kilogram that accounts for the
						  chemical accumulation in aquatic organisms from all potential exposure routes,
						  including water, food and sediment.
(c) The Department will establish criteria
						  for threshold toxics in accordance with the following guidelines:
(1) If the EPA has developed criteria, the
						  Department will evaluate and accept the criteria when it is determined that
						  they are adequate to protect the designated water uses.
(2) If the EPA criteria have been evaluated,
						  and have been determined to be inadequate to protect designated uses, or when
						  no criteria have been developed for a substance identified or expected in a
						  discharge, the Department will develop criteria following EPA's standard
						  toxicological procedures outlined in the Methodology for Deriving Ambient Water
						  Quality Criteria for the Protection of Human Health (EPA-822-B-00-004, October
						  2000), as amended and updated.
(3)
						  If no data are available to characterize the human health hazard of a chemical,
						  no criterion will be developed. A criterion to protect the next most sensitive
						  use will be used. A threshold criterion will be developed at a future date if
						  information becomes available.