Forest Trail


23 results found for



A colorless, naturally occurring, radioactive, inert gaseous element formed by radioactive decay of radium atoms in soil or rocks.

Reactive Organic Gas (ROG)

A photochemically reactive chemical gas, composed of non-methane hydrocarbons, that may contribute to the formation of smog. Also sometimes referred to as Non-Methane Organic Gases (NMOGs). (See also Volatile Organic Compounds and Hydrocarbons.)

Reactivity (or Hydrocarbon Photochemical Reactivity)

A term used in the context of air quality management to describe a hydrocarbon's ability to react (participate in photochemical reactions) to form ozone in the atmosphere. Different hydrocarbons react at different rates. The more reactive a hydrocarbon, the greater potential it has to form ozone.

Reasonably Available Control Measures (RACM)

A broadly defined term referring to technologies and other measures that can be used to control pollution. They include Reasonably Available Control Technology and other measures. In the case of PM10, RACM refers to approaches for controlling small or dispersed source categories such as road dust, woodstoves and open burning.

Reasonably Available Control Technology (RACT)

Control techniques defined in U.S. EPA guidelines for limiting emissions from existing sources in nonattainment areas. RACTs are adopted and implemented by states. For more information, visit our RACT website.

Reasonably Available Retrofit Control Technology (RARCT)

(See also Best Available Control Technology.)

Reciprocating Internal Combustion Engine

An engine in which air and fuel are introduced into cylinders, compressed by pistons and ignited by a spark plug or by compression. Combustion in the cylinders pushes the pistons sequentially, transferring energy to the crankshaft, causing it to rotate.

Reference Dose (RfD)

An estimate delivered by the U.S. EPA (with uncertainty spanning perhaps an order of magnitude) of the daily exposure to the human population, (including sensitive subpopulations) that is likely to be without deleterious effects during a lifetime. The RfD is reported in units of mg of substance/kg body weight/day for oral exposures.

Reference Exposure Concentration (RfC)

An estimate, derived by the U.S. EPA with an uncertainty spanning perhaps an order of magnitude) of a daily exposure to the human population, (including sensitive subgroups) that is likely to be without appreciable risk of deleterious effects during a lifetime of exposure. The RfC is derived from a no or lowest observed adverse effect level from human or animal exposures, to which uncertainty or "safety" factors are applied.

Reference Exposure Level (REL)

A term used in risk assessment. It is the concentration at or below which no adverse health effects are anticipated for a specified exposure period.


A facility that produces liquid fuels by distilling petroleum.


Establishing native tree cover on lands that were previously forested, but that have had less than 10 percent tree canopy cover for a minimum time of 10 years.

Reformulated Gasoline (RFG)

Also called Cleaner Burning Gasoline (CBG). Gasoline with a different composition from conventional gasoline (e.g., lower aromatics content) that results in the production of lower levels of air pollutants. For more information, see our cleaner burning gasoline website.

Reformulated Gasoline Predictive Model

A set of mathematical equations that predict the emissions likely to occur from the combustion of a given formulation of gasoline. For more information, see our predictive model website area.

Regional Haze

The haze produced by a multitude of sources and activities which emit fine particles and their precursors across a broad geographic area. National regulations require states to develop plans to reduce the regional haze that impairs visibility in national parks and wilderness areas.

Registration Stop (or VLT Stop)

A hold placed on a motor vehicle or motor vehicle engine to prevent registration or title being issued. Registration stops may be placed by DMV, ARB or CHP staff. Stops are placed because of illegal registration attempts, illegal sales, or placed on heavy-duty vehicles that have been retired by their previous owners. Registration stops do not expire; however, a vehicle with a stop may be dropped from the DMV computer record after 7-10 years.

Reid Vapor Pressure (RVP)

Refers to the vapor pressure of the fuel expressed in the nearest hundredth of a pound per square inch (psi) with a higher number reflecting more gasoline evaporation. (See also Gasoline Volatility.)

Renewable Fuel Standard (RFS)

A federal program to increase the volume of renewable fuels used in transportation fuels. Created under the Energy Policy Act of 2005, and revised by the Energy Independence and Security Act of 2007, the RFS program requires increasing annual volumes of renewable fuel, starting from 9 billion gallons in 2008 to 36 billion gallons by 2022. Within those total volumes, the RFS also requires certain volumes of specific fuels, such as cellulosic and advanced biofuels.

Research Screening Committee (RSC)

The Board's legislatively mandated committee consists of scientists, engineers and others knowledgeable, technically qualified and experienced in air pollution problems. The committee meets approximately four times a year to review proposed and completed research projects.

Residual Risk

The quantity of health risk remaining after application of emission control.

Ringelmann Chart

A series of charts, numbered 0 to 5, that simulate various smoke densities by presenting different percentages of black. A Ringelmann No. 1 is equivalent to 20 percent black; a Ringelmann No. 5 is 100 percent black. They are used for measuring the opacity or equivalent obscuration of smoke arising from stacks and other sources by matching the actual effluent with the various numbers, or densities, indicated by the charts.

Risk Assessment

An evaluation of risk which estimates the relationship between exposure to a harmful substance and the likelihood that harm will result from that exposure.

Risk Management

An evaluation of the need for and feasibility of reducing risk. It includes consideration of magnitude of risk, available control technologies and economic feasibility.