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

(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:

(d) The sources the Department uses to obtain relevant risk assessment values for protection for threshold level toxic effects to human health are as follows: