PBT (Persistent, Bioaccumulative and Toxic)
The US Environmental Protection Agency (EPA) designated PBTs to be higher hazard substances in 1999, which meant that the Massachusetts list was also changed.
A definition of persistent, bioaccumulative and toxic (“PBT”) chemicals can be found at the U.S. EPA's PBT Profiler website's Here is a summary definition of these terms.
Persistence is the ability of a chemical substance to remain in an environment in an unchanged form. The persistance is expressed in individual medium half-lives in air, water, soil, and sediment measured in days. (Half-life is the length of time it takes for the concentration of a substance to be reduced by one-half relative to its initial level, assuming first-order decay kinetics. Generally, it is useful to consider "complete removal" as taking approximately six half-lives.) For example, for a given chemical in air, it is classified as Persistant if half of it is still around after 2 days.
Bioaccumulation is the process by which the chemical concentration in an aquatic organism achieves a level that exceeds that in the water, as a result of chemical uptake through all possible routes of exposure. The EPA uses the bioconcentration in fish as its bioconcentration factor, or BCF. The bioconcentration factor (BCF) is a measure of the ability for a water-borne chemical substance to concentrate in fatty tissue of fish and aquatic organisms relative to its surroundings. EPA defines bioconcentration as the net accumulation of a substance by an aquatic organism as a result of uptake directly from the ambient water through gill membranes or other external body surfaces. For a chemical, it's classified as Bioaccumulative if the BCF >= 1,000.
Toxicity is a relative property of a chemical that refers to its potential to have a harmful effect on a living organism. It is a function of the concentration of the chemical and the duration of exposure. A chronic (long-term) toxicity value called a ChV (defined below) estimates a chemical's relative toxicity. The Chronic Value (ChV) is defined as the geometric mean of the no observed effect concentration (NOEC) and the lowest observed effect concentration (LOEC) in a fish. This can be mathematically represented as: ChV = 10^([log (LOEC x NOEC)]/2). For a chemical, it's toxic if ChV <= 10 mg.