Forest Trail


32 results found for


Partial Zero Emission Vehicle (PZEV)

Vehicles that meet the ARB super ultra low emissions standard, have zero evaporative emissions and have a 15 year/150,000 mile warranty. See our Drive Clean website.

Particulate Matter (PM)

Any material, except pure water, that exists in the solid or liquid state in the atmosphere. The size of particulate matter can vary from coarse, wind-blown dust particles to fine particle combustion products. For more information, see ARB's PM brochure.

Peak Levels

A level of airborne pollutants that is much higher than average. They can occur over a short period of minutes or hours in response to sudden releases, or they can occur due to a longer term build-up over several days.

Perchloroethylene (Perc)

The substance with the chemical formula 'C2Cl4,' also known by the name 'tetrachloroethylene' which has been identified by the ARB and listed as a toxic air contaminant (title 17, California Code of Regulations, section 93000).

Periodic Smoke Inspection Program (PSIP)

Regulation requiring fleet owners of two or more heavy-duty diesel powered trucks or buses to perform annual smoke opacity inspections on each vehicle's engine that is four years old or older. Engines that exceed opacity standards must be repaired to be brought into compliance. Fleet owners must keep records of the annual smoke test for two years and make these records available to ARB upon request.


Written authorization from a government agency (e.g., an air quality management district) that allows for the construction and/or operation of an emissions generating facility or its equipment within certain specified limits. For more information, visit our permitting activities portal page.

Permit to Operate (P/O)

An operational permit issued yearly by an air district to sources that meet specified regulations.

Peroxyacytal Nitrate (PAN)

A group of compounds formed from the photochemical reactions of nitrogen and organic compounds. PANs are components of smog and known to cause eye irritation.


Refers to the length of time a compound stays in the atmosphere, once introduced. A compound may persist for less than a second or indefinitely.

Personal Watercraft (PWC)

Watercraft that do not have outboard, inboard, or stern drive engines. This encompasses the watercraft typically referred to as Jet Skis, Waverunners, etc. For more information, see our recreational marine website.

Photochemical Reaction

A term referring to chemical reactions brought about by the light energy of the sun. The reaction of nitrogen oxides with hydrocarbons in the presence of sunlight to form ozone is an example of a photochemical reaction.


Chemical decomposition induced by light or other energy.

Plug-in Hybrid Electric Vehicle (PHEV)

A vehicle that is similar to traditional hybrids but is also equipped with a larger, more advanced battery that allows the vehicle to be plugged in and recharged in addition to refueling with gasoline. This larger battery allows you to drive on a combination of electric and gasoline fuels. See our Drive Clean website.

A visible or measurable discharge of a contaminant from a given point of origin that can be measured according to the Ringelmann scale. (See Ringelmann Chart.)
PM10 (Particulate Matter)

A criteria air pollutant consisting of small particles with an aerodynamic diameter less than or equal to a nominal 10 microns (about 1/7 the diameter of a single human hair). Their small size allows them to make their way to the air sacs deep within the lungs where they may be deposited and result in adverse health effects. PM10 also causes visibility reduction. For more information, see our particulate matter brochure.


Includes tiny particles with an aerodynamic diameter less than or equal to a nominal 2.5 microns. This fraction of particulate matter penetrates most deeply into the lungs. For more information, see our particulate matter website.

Point Sources

Specific points of origin where pollutants are emitted into the atmosphere such as factory smokestacks. (See also Area-Wide Sources and Fugitive Emissions.)

Pollutant Standards Index (PSI)

A numerical index formerly used for reporting severity of air pollution levels to the general public. The PSI incorporated the five criteria pollutants -- ozone, PM10, carbon monoxide, sulfur dioxide and nitrogen dioxide -- into one single index. The PSI was based on the 1-hour ozone standard. PSI levels ranged from 0 (Good air quality) to 500 (Hazardous air quality). The higher the index, the higher the level of pollutants and the greater likelihood of health effects. For more information, see our Air Quality Index website.

Pollution Prevention

The use of materials, processes, or practices to reduce, minimize, or eliminate the creation of pollutants or wastes. It includes practices that reduce the use of toxic or hazardous materials, energy, water and/or other resources. For more information, see our Pollution Prevention Program website.

Polycyclic Aromatic Hydrocarbons (PAHs)

Organic compounds which include only carbon and hydrogen with a fused ring structure containing at least two benzene (six-sided) rings. PAHs may also contain additional fused rings that are not six-sided. The combustion of organic substances is a common source of atmospheric PAHs.


Natural or synthetic chemical compounds composed of up to millions of repeated linked units, each of a relatively light and simple molecule.

Positive Crankcase Ventilation (PCV)
An emission control system for a reciprocating internal combustion engine that involves recirculating gases that blow by the piston rings during combustion from the crankcase back into the intake manifold so they can be more completely burned.

Pollution control device that collects particles from an air stream. (See Electrostatic Precipitator.)

Prescribed Burning

The planned application of fire to vegetation to achieve any specific objective on lands selected in advance of that application. In California, prescribed burning is governed under the Agricultural Burning Guidelines.

Prevention of Significant Deterioration (PSD)

A permitting program for new and modified stationary sources of air pollution located in an area that attains or is unclassified for national ambient air quality standards (NAAQS). The PSD program is designed to ensure that air quality does not degrade beyond those air quality standards or beyond specified incremental amounts. The PSD permitting process requires new and modified facilities above a specified size threshold to be carefully reviewed prior to construction for air quality impacts. PSD also requires those facilities to apply BACT to minimize emissions of air pollutants. A public notification process is conducted prior to issuance of final PSD permits.

Primary Particles

Particles that are directly emitted from combustion and fugitive dust sources. (Compare with Secondary Particle.)


A gas with a high vapor pressure used to force formulations out of aerosol spray cans. Among the gases used are butanes, propanes and nitrogen.

Proposition 65

Safe Drinking and Toxic Enforcement Act of 1986, also known as Proposition 65. This act is codified in California Health and Safety Code section 25249.5, et seq. No person in the course of doing business shall knowingly discharge or release a chemical known to the state to cause cancer or reproductive toxicity into water or into land where such chemical passes or probably will pass into any source of drinking water, without first giving clear and reasonable warning to such individual. For more information, visit the OEHHA's Prop 65 website.

Public Owned Utilities (POUs)

Non-profit utility providers owned by a community and operated by municipalities, counties, states, public power districts, or other public organizations. Within POUs, residents have a say in decisions and policies about rates, services, generating fuels and the environment.

Public Workshop

A workshop held by a public agency for the purpose of informing the public and obtaining its input on the development of a regulatory action or control measure by that agency.

Publicly Owned Treatment Works (POTW)

Publicly Owned Treatment Works (POTW) are facilities designed to collect, transmit and treat wastewater that may be generated by industrial, commercial and/or domestic sources. Treatment works include the wastewater treatment units themselves, as well as intercepting sewers, outfall sewers, sewage collection systems, pumping, power and other equipment.