Science Terms Definitions - P

Particle - A tiny mass of material. Airborne particles, materials that exist in the atmosphere as a solid or liquid, can be natural, caused by stirring of soil dusts, or anthropogenic. They vary in size from coarse (diameter > 3 µm) to fine (< 3µm) . Sometimes inhalable or respirable is used to describe those particles (< 2 µm) which can be inhaled through the nose and enter the lungs.

Particulates - Fine liquid or solid particles such as dust, smoke, mist, fumes, or smog, found in the air or emissions.

Partition Coefficient - Measure of the sorption phenomenon, whereby a pesticide is divided between the soil and water phase; also referred to as adsorption partition coefficient.

Perched Water - Zone of unpressurized water held above the water table by impermeable rock or sediment.

Percolation - Downward flow or filtering of water through pores or spaces in rock or soil.

Performance Data (for incinerators) - Information collected, during a trial burn, on concentrations of designated organic compounds and pollutants found in incinerator emissions. Data analysis must show that the incinerator meets performance standards under operating conditions specified in the RCRA permit.

Performance Standards

(1) Regulatory requirements limiting the concentrations of designated organic compounds, particulate matter, and hydrogen chloride in emissions from incinerators.

(2) Operating standards established by EPA for various permitted pollution control systems, asbestos inspections, and various program operations and maintenance requirements.

Person-rem - Used as a unit of population dose; the average dose per individual expressed in rems times the population affected.

Person-year - The sum of the number of years each person in the study population is at risk; a metric used to aggregate the total population at risk assuming that 10 people at risk for one year is equivalent to 1 person at risk for 10 years.

pH - A measure of the acidity or alkalinity of a material, liquid or solid (pH is represented on a scale of 0 to 14 with 7 representing a neutral state, 0 representing the most acid, and 14 the most alkaline).

Photon - The indivisible unit, or quantum, of electromagnetic radiation. The energy of the photons determines the nature of the radiation, from radio waves at the lowest energy levels, up through infra-red, visible, and ultra-violet light, to X-or gamma-rays, which have energy high enough to ionize atoms.

Picocurie - One trillionth (10-12) of a curie.

Plume - A visible or measurable discharge of a contaminant from a given point of origin. Can be visible or thermal in water as it extends downstream from the pollution source, or visible in air as, for example, a plume of smoke. The area of radiation leaking from a damaged reactor. Area downwind within which a release could be dangerous for those exposed to leaking fumes.

Plutonium - A highly toxic, heavy, radioactive metallic element. There are 15 isotopes of plutonium, of which only five are produced in significant quantities: plutonium-238, -239, -240, -241, and -242. Plutonium-239 is the most important plutonium isotope as it is fissile and is used in nuclear weapons and some reactors. One the other hand, plutonium-240 is unsuitable for use in nuclear weapons and reactor fuel. Thus, in a reactor whose main purpose is plutonium production, the rate at which plutonium-240 is formed controls the length of time fuel is allowed to remain under irradiation. Plutonium is categorized according to plutonium-240 content, as follows: super-grade has 2-3% Pu-240; weapons-grade has less than 7% Pu-240; fuel-grade has 7-18 (or sometimes given as 7-19) % Pu-240; and reactor-grade has 18 or greater (or 19 or greater) % Pu-240. (Note: Despite what the name implies, "reactor-grade" plutonium has been used successfully to make a nuclear bomb.)

Point Source - A single isolated stationary source of pollution.

Pollutant - Any material entering the environment that has undesired effects.

Pollution - The presence of matter or energy whose nature, location or quantity produces undesired environmental effects.

Population at Risk - A limited population that may be unique for a specific dose-effect relationship; the uniqueness may be with respect to susceptibility to the effect or with respect to the dose or exposure itself.

Population Dose (population exposure) - The summation of individual radiation doses received by all those exposed to the source or event being considered.

Positron - An elementary particle with a positive electric charge, but in other respects identical with an electron.

Preliminary Assessment - The process of collecting and reviewing available information about a known or suspected waste site or release.

Primary Drinking Water Regulation - Applies to public water systems and specifies a contaminant level, which, in the judgment of the EPA Administrator, will not adversely affect human health.

Probability - The chance that a particular event will occur given the population of all possible events. See definition for risk.

Probable Error - The magnitude of error which is estimated to have been made in determination of results.

Probit Analysis - A statistical transformation which will make the cumulative normal distribution linear. In analysis of dose-response, when the data on response rate as a function of dose are given as probits, the linear regression line of these data yields the best estimate of the dose-response curve. The probit unit is y = 5 + Z(p) , where p = the prevalence of response at each dose level and Z(p) = the corresponding value of the standard cumulative normal distribution.

Process Wastes - Any designated toxic pollutant or combination of pollutants, whether in wastewater or otherwise present, which is inherent to or unavoidable resulting from any manufacturing process, including that which comes into direct contact with or results from the production or use of any raw material, intermediate product, finished product, byproduct or waste product and is discharged into the navigable waters.

Proportionate Mortality Ratio (PMR) - The fraction of all deaths from a given cause in the study population divided by the same fraction from a standard population. A tool for investigating cause-specific risks when only data on deaths are available. If data on the population at risk are also available, SMRs are preferred.

Prospective Study - An inquiry in which groups of individuals are selected in terms of whether they are or are not exposed to certain factors, and then followed over time to determine differences in the rate at which disease develops in relation to exposure to the factor. Also called cohort study.

Proton - An elementary particle with a positive electric charge and a mass that is given the value 1 on the scale of atomic weights.

Public Comment Period - The time allowed for the public to express its views and concerns regarding an action by EPA (e.g., a Federal Register Notice of proposed rule-making, a public notice of a draft permit, or a Notice of Intent to Deny).

Public Hearing - A formal meeting wherein EPA officials hear the public's views and concerns about an EPA action or proposal. EPA is required to consider such comments when evaluating its actions. Public hearings must be held upon request during the public comment period.

Public Notice

1. Notification by EPA informing the public of Agency actions such as the issuance of a draft permit or scheduling of a hearing. EPA is required to ensure proper public notice, including publication in newspapers and broadcast over radio stations.

2. In the safe drinking water program, water suppliers are required to publish and broadcast notices when pollution problems are discovered.

Pyrophoric Uranium - A chemical that will ignite spontaneously in air at or below a temperature of 130 Fahrenheit (54.5 C). Uranium metal is pyrophoric especially when finely divided. This is a fire hazard as any pyrophoric substance can spontaneously self-ignite when exposed to normal atmospheric conditions. Uranium and its compounds are highly toxic, both from a chemical and radiological standpoint.