Haz-Map Chemical, Biological & Nuclear Glossary


A chemical that accelerates a chemical reaction in the production of rubber or plastics.

Acceptable Daily Intake (ADI)

"Daily intake of a chemical (e.g., food additive, pesticide) that, during the entire lifetime, appears to be without appreciable risk (affects 1 in 1 million people or less) on the basis of all known  facts at the time." ADIs or RfDs are typically based on either NOAELs or LOAELs from epidemiology or animal toxicology studies. Uncertainty factors are then applied. [Hayes, pp. 2155, 70]


American Conference of Governmental Industrial Hygienists.

ACGIH Carcinogen

1 = Confirmed Human; 2 = Suspected Human; 3 = Confirmed Animal; 4 = Not Classifiable; 5 = Not Suspected;

acute-moderate disease

The onset is acute, but not life threatening. For chemicals, the length of exposure is less than 24 hours. The patient is likely to seek medical attention, but not be admitted to the hospital.

acute-severe disease

The onset is acute, severe, and life threatening. For chemicals, the length of exposure is less than 24 hours. The patient is likely to be admitted to the hospital.


Small particles, usually in the range of 0.01 to 100 micrometers, dispersed in air; includes liquid (mist) and solid particles (dust).


The time, in the range of 2 minutes to 2 days, required for irreversible binding between organophosphate insecticides or military nerve agents and the acetylcholinesterase enzyme. If the antidote pralidoxime is not given before aging occurs, the patient may remain paralyzed for weeks on mechanical ventilation. [AHLS, p. 300-312]


American Industrial Hygiene Association.

alpha particle

"Nucleus of a helium atom emitted by certain radioisotopes upon disintegration. Contains two protons and two neutrons." [Gusev, p. 541] "Alpha particles up to 7.5 MeV are stopped in the dead layer of normal human skin." [Gollnick, p. 74]


Tiny sacs at the ends of bronchioles in the lungs; oxygen and carbon dioxide gas exchange takes place here with red blood cells in adjacent capillaries.


Decreased hemoglobin or number of red blood cells.


Temporary loss of consciousness induced by high concentrations of organic solvents.


A drug used to inhibit or kill microorganisms.


Proteins produced by the body's immune system in response to specific antigens.


A foreign substance that can induce the production of antibodies in the body.

Annual Limit on Intake (ALI)

"The activity of a radionuclide which, when taken alone, would irradiate a person to the limit set for each year of occupational exposure." [Gusev, p. 541] For values, see NRC ALIs.

aplastic anemia

One type of anemia caused by injury to blood forming tissues and associated with occupational exposure to TNT, benzene, and ionizing radiation.


An organic chemical (hydrocarbon) characterized by the presence of a benzene ring.


Reversible bronchoconstriction (narrowing of bronchioles) initiated by the inhalation of irritating or allergenic agents.


  1. Refers to a tumor that does not invade other tissues and does not metastasize to other parts of the body. 2) Refers to an interstitial lung disease without any apparent symptoms or detectable changes in pulmonary function, e.g., siderosis caused by iron oxide.

beta particle

"An electron of positive or negative charge." [Gusev, p. 542] Beta particles will penetrate about 4 meters of air per MeV of energy. Beta particles will penetrate about 0.5 cm in soft tissue per MeV of energy. Beta particles up to 70 keV are stopped in the dead layer of normal human skin." [Gollnick, p. 74]


Progressive increase of a poison in the body; occurs because the rate of intake exceeds the rate of elimination. "The process by which a material in an organism's environment progressively concentrates within the organism." NTP Glossary


Biological Exposure Indices (ACGIH)

bremsstrahlung radiation

"Secondary photon radiation (x-ray) produced by the deceleration of charged particles through matter. Usually associated with energetic beta emitters, e.g., P-32." ORCBS


Bronchioles are the narrowest airways that branch from the bronchi of the trachea.


Chronic bronchitis is persistent coughing and production of phlegm for at least 3 months out of the year for at least two successive years. (American Thoracic Society)


Burns are characterized as: 1st degree-- redness; 2nd degree-- blisters; and 3rd degree--ulcers that heal by scarring.


The carbamate insecticides interrupt nerve conduction. They cause the accumulation of acetylcholine at nerve endings by reversibly binding with the acetylcholinesterase enzyme.


A chemical that can increase the incidence of cancer in exposed populations. Chemicals are classified by the International Agency for Research on Cancer (IARC) as known; probable, or possible human carcinogens based on available epidemiologic and toxicological evidence. "Exposure to carcinogens must be kept to a minimum. Workers exposed to A1 carcinogens without a TLV should be properly equipped to eliminate to the fullest extent possible all exposure to the carcinogen. For A2 and A3 carcinogens without a TLV, worker exposure by all routes should be carefully controlled." [ACGIH]


Chemical Abstracts Service registry number, a unique number for each chemical in the format xxx-xx-x.


Chronic Beryllium Disease.


"The concentration that should not be exceeded during any part of the working exposure." (ACGIH) See "threshold limit value."


Code of Federal Regulations.

Chemical Asphyxiant

A poison that blocks either the transport or use of oxygen by living organisms.


A skin disease characterized by acne that is caused by exposure to dioxin, pentachlorophenol, PCBs, and other chlorinated hydrocarbon compounds.

chronic disease

The onset of symptoms is gradual over a period longer than 2 months. A chronic disease induced by chemicals may represent 1) A cumulative exposure with a long latency, or 2) Adverse effects that persist two months or longer after a brief high exposure.


Central Nervous System: the brain and spinal cord.

CNS solvent syndrome

Organic solvents can affect the central nervous system both acutely (increased reaction time and anesthesia) and chronically (permanent brain damage).

confined space

"A space that is large enough and so configured that an employee can bodily enter and perform work. Confined spaces have limited or restricted means for entry or exit and are not designed for continuous occupancy. Confined spaces include storage tanks, bins, boilers, ventilation and exhaust ducts, pits, manholes, vats, and reactor vessels." [DOE OSH Technical Reference]

contact dermatitis

Dermatitis caused by contact with irritating or allergenic substances.

counts per minute (cpm)

"The number of counts or nuclear events detected by a radiation survey device such as a Geiger counter. Since not all events that occur are detected, cpm are always less than actual disintegrations per minute (dpm) emanating from a radioactive material." [Gusev, p. 544]


Pertaining to chemicals that can induce severe burns at the site of contact.

Critical Organ

"That part of the body that is most susceptible to radiation damage under the specific conditions under consideration." [NRC Glossary] See Table A4.5 in Gusev.


Cerebrospinal fluid.


Coal Workers’ Pneumoconiosis.

daughter radionuclide

"Decay product produced by a radionuclide. The element from which the daughter was produced is called the 'parent.'" [Gusev, p. 544]


"Absorbs water from air and forms wet solid or solution." [CHEMINFO] "Deliquescent" refers to chemical salts that dissolve in water from the air, while "hygroscopic" refers to substances including cotton and paper fabrics that absorb water from the air. [Hawley]


A chemical that can cause skin disease.

developmental effects

"Adverse effects such as altered growth, structural abnormality, functional deficiency, or death observed in a developing organism." [EPA Glossary]


"A chemical that destroys vegetative forms of harmful microorganisms, but does not ordinarily kill bacterial spores." [EPA]


Diffusing capacity of the lung for carbon monoxide. A nonspecific test that can be abnormal in restrictive, obstructive, or vascular diseases of the lung. [LaDou, p. 312] "The DLCO provides a relatively good approximation of gas exchange abnormalities caused by ventilation-perfusion mismatching, venous admixture (shunting), and barriers to diffusion of gases. But it does not separate among these causes." [Sullivan, p. 216] "DCO is primarily of value for persons with restrictive lung disease." [AMA Guides, 5th Ed., p. 107]

dose-response relationship

Relationship between the dose of a toxic chemical and the incidence of an adverse effect. This is a fundamental law of toxicology expressed as, "The dose makes the poison." For any poison, there exists a threshold dose below which adverse effects do not occur.


Increased intercellular fluid in tissues.


The formation of a powdery surface on crystals, as a hydrate is converted to anhydrous form by loss of combined water molecules to the atmosphere; [Hawley]


Emergency Exposure Guidance Level. (NRC).


A plastic or synthetic rubber with elastic properties at room temperature.

electron capture

"Method of radioactive decay in which the nucleus captures an orbital electron, which then interacts with a proton, effectively negating the proton and transmuting the nucleus to that of another element." [Gusev, p. 546] "Electron capture will occur when there are too many protons in the nucleus, and there isn't enough energy to emit a positron." EPA website: Understanding Radiation

engineering controls

Methods put in place (engineered) to control the source of worker exposure, e.g., exhaust ventilation systems or glove-box enclosures.


Environmental Protection Agency.


1 hr exposure limit: 1 = mild transient health effects or objectionable odor; 2 = impaired ability to take protective action; 3 = life threatening health effects; [Emergency Response Planning Guidelines, AIHA]

Explosive Polymerization

". . . a material that may polymerize violently under high temperature conditions or contamination with other products. It is also used to identify materials that have a strong potential for polymerization in the absence of an inhibitor due to depletion of this inhibitor caused by accident conditions." [ERG 2016]


Inducing tissue injury and fibrosis (scarring).

fissile material

"Although sometimes used as a synonym  for fissionable material, this term has acquired a more restricted meaning. Namely, any material fissionable by thermal (slow) neutrons. The three primary fissile materials are uranium-233, uranium-235, and plutonium-239." NRC Glossary


NFPA flammability code: 0 = will not burn; 1 = must be preheated; 2 = high ambient temp required; 3 = may ignite at ambient temp; 4 = burn readily. See PubChem site for updates on NFPA Fire Ratings and GHS Classification. Also see full definitions of NFPA Flammability Codes.

Flash Point

"The minimum temperature at which a substance gives off flammable vapor(s). Substance will ignite when coming in contact with a spark or flame. [How to Use the HMR]


"A material that is prohibited from being offered or accepted for transportation. This prohibition does not apply if these materials are: diluted, stabilized, or incorporated into devices and classed in accordance with Part 173." [How to Use the HMR]

gamma ray

"Radiation emitted from the nucleus having a wavelength range from 10-9 to 10-12 cm." [Gusev, p. 547] "Gamma rays originate in the nucleus as a means of releasing excess energy from the atom, while X-rays are emitted when electrons outside the nucleus move from higher to lower energy states. Radionuclides having gamma-ray energies less than 0.03 MeV per decay, considering the fractional yield (as described above), generally do not present a health concern from external gamma exposure." Argonne

Gamma Ray Constant

"The specific gamma ray constant can be used to find the exposure rate (R/h) for a source of activity A (mCi) at any distance d (cm) by using the following formula: Exposure rate = Gamma ray constant X A / d2." [Gusev, p. 588] For values, see Vanderbilt University Online Radiation Safety


Pertaining to the organs of the digestive system, including the stomach, liver, and intestines.

Geiger-Mueller counter

"Can be used to detect gamma and X rays as well as energetic beta particles. Cannot detect alpha particles. Generally this instrument is used to detect low radiation levels and should not be used in radiation fields >500 mR/h." [Gusev, p. 547]


Time required to reduce by one half the amount of a chemical absorbed by the body. Half-life can be calculated accurately only for those substances eliminated linearly, independent of concentration. For linearly eliminated substances, it takes approximately 3.5 half-lives to eliminate 90% of the substance. [LaDou, p.183]

Half-Life, Radiological

"The time in which one half of the atoms of a particular radioactive substance disintegrate into another nuclear form." NRC Glossary "Only those radionuclides with half-lives longer than about one year are of concern for DOE environmental management sites, as shorter-lived radionuclides will have already decayed away to innocuous levels because production activities involving radioactive materials at major sites such as Hanford ceased more than ten years ago." [Argonne]

Half-Life, Effective

"The time required for a radionuclide contained in a biological system, such as a human or an animal, to reduce its activity by one-half as a combined result of radioactive decay and biological elimination." NRC Glossary For values, see Gusev, Appendix 6.

hard metal disease

Interstitial lung disease caused by repeated inhalation of cobalt dusts or fume. Exposure occurs during manufacturing or use of hard metal tools containing cobalt/tungsten carbide.


A chemical that is toxic to blood or blood-forming tissues.


Proteins in red blood cells that carry oxygen.

hemolytic anemia

Anemia induced by chemicals that lyse red blood cells.


Primary hepatotoxins: the toxic effect to the liver is the principal adverse effect of the chemical. Secondary hepatotoxins: the potential for toxic effect in the occupational setting is based on cases of poisoning by human ingestion or animal experimentation.


Pertaining to the microscopic study of diseased cells and tissues.

High-resolution computerized tomography (HRCT)

An x-ray procedure used to diagnose lung diseases.

hypersensitivity pneumonitis

An interstitial lung disease caused by repeated inhalation of moldy organic dusts, e.g., Farmer’s lung.


Deficiency of oxygen available to living tissues.


International Agency for Research on Cancer; classifies chemicals as established (1), probable (2a), or possible (2b) human carcinogens, or (3) not classifiable.


Immediately Dangerous to Life or Health. (NIOSH)

inhalation fever

An acute, flu-like illness that begins within a few hours of inhalation exposure to high concentrations of the causative dust or mist, e.g., zinc oxide fumes (metal fume fever) or organic dusts (organic dust toxic syndrome).   

interstitial pulmonary fibrosis

Asbestos, coal, and silica dusts induce scar formation in interstitial lung tissue. The interstitium is the supporting lung tissue adjacent to pulmonary airways and blood vessels.


Chemicals with the same composition and molecular weight but different physical or chemical properties.


"Isotopes are different forms of an element that have the same number of protons in the nucleus but a different number of neutrons." [Argonne]

lacrimator  (lachrymator)

A substance that irritates the eyes and induces the flow of tears.


The latency for an occupational disease is the time lag between exposure to the toxin and detection of the disease.

Lethal Concentration (LC)

LC50 is the lethal concentration in 50% of animals tested in an inhalation experiment for a given time (usually 1-4 hours). LCLo is the lowest lethal concentration tested in animal inhalation experiments. Most of the data in Haz-Map comes from ChemIDplus.

Lethal Dose 50 (LD50)

"The dose of a toxicant that will kill 50% of test organisms within a designated period of time. The lower the LD 50, the more toxic the compound." [EPA] The dose is administered by any route other than inhalation.

Lower Explosive Limit (LEL)

The minimum concentration below which combustion will not occur.

Lowest-Observed-Adverse-Effect Level (LOAEL)

"The lowest dose in a toxicity study resulting in adverse health effects." [EPA]

lung cancer

Cancer of the lung caused by occupational exposures—all of these agents are in the IARC Group 1 (known human carcinogens).

Lung Clearance Half-time

"The ALIs and DACs for inhalation are given for an aerosol with an activity median aerodynamic diameter (AMAD) of 1 um and for three classes (D,W,Y) of radioactive material, which refer to their retention  (approximately days, weeks or years) in the pulmonary region of the lung. This classification applies to a range of clearance half-times of less than 10 days for D, for W from 10 to 100 days, and for Y greater than 100 days." [Appendix B to Part 20] For values, see NRC ALIs.


Maximum Allowable Concentration from German Research Foundation (DFG).


The presence of increased methemoglobin in the blood; chemicals are classified as either Primary (Methemoglobinemia is the primary toxic effect.) or Secondary.


Milligrams per cubic meter of air.


Millimeters of mercury; at sea level the earth's atmosphere exerts a pressure of 760 mmHg. 1 mmHg = 1 Torr.


Millions of particles per cubic foot of air.


Material Safety Data Sheet.


Mine Safety and Health Administration.


"A substance or agent capable of altering the genetic material in a living cell." [Luttrell, p. 417] "In fact, most chemicals classified as noncarcinogens by agencies such as the National Toxicology Program (NTP) and International Agency for Research on Cancer (IARC) produced positive responses in one or more of a battery on in vitro genetic assays, suggesting that the specificity of in vitro tests is too poor to rely upon individual tests or test batteries to predict carcinogenic profiles for new molecules. Subsequent addition of in vivo test results (e.g., mouse micronucleus) to the battery database failed to improve its predictive performance. . . . With the concordance between rodent carcinogens and mutagens averaging about 60 to 70 percent, the appropriate integration of genetic toxicology results into  toxicological assessments is not a straight-forward process." [Hayes, p. 1188-90]


Temporary sleepiness induced by high concentrations of organic solvents.

NCRP (National Council on Radiation Protection and Measurements) 

"Governmental organizations, including the Nuclear Regulatory Commission, the Public Health Service, the Environmental Protection Agency and state governments utilize NCRP's recommendations as the scientific basis of their radiation protection activities." NCRP


The chemical is potentially toxic to the kidneys in the occupational setting.


Impairment of sensory and/or motor nerve function.


The chemical is toxic to nerve cells in the occupational setting—includes peripheral neuropathy (predominantly motor or sensorimotor), Parkinsonism, solvent syndrome (acute or chronic), and other CNS neurotoxins. [Rom, p. 697-707; LaDou, p. 386-96]


National Fire Protection Agency. See “Flammability.”


National Institute for Occupational Safety and Health.

No-Observed-Adverse-Effect Level (NOAEL)

"The dose of a chemical at which there were no statistically or biologically significant increases in frequency or severity of adverse effects seen between the exposed population and its appropriate control." [ATSDR]


National Research Council;

NRC (Nuclear Regulatory Commission)

U.S. government agency regulating by-product material (radioactive material arising from controlled fission). [Gusev, p. 542, 551]


National Toxicology Program

NTP Carcinogen

K = Known to be a human carcinogen; R = Reasonably anticipated to be a human carcinogen;

olfactory fatigue

A person's ability to detect the odor of a chemical declines as exposure time increases. [AHLS, p. 46]

odor threshold

The lowest concentration at which a substance can be detected or recognized using the sense of smell.


The organochlorine insecticides, including DDT and chlordane, persist in the environment and in animal tissues.


Organophosphate insecticides interrupt nerve conduction. They cause the accumulation of acetylcholine at nerve endings by irreversibly binding with the acetylcholinesterase enzyme.


Occupational Safety and Health Administration.

other CNS neurotoxin

In addition to chemicals that cause neuropathy, Parkinson’s syndrome, and CNS solvent syndrome, other chemicals that cause acute encephalopathy include organic metal compounds (lead, mercury, tin, nickel, and manganese), hydrazines, boron hydrides, etc. 


Tingling or numbness.

Parkinson’s disease

A degenerative central nervous system disorder with 4 characteristic features: slowness and poverty of movement, muscular rigidity, resting tremor, and postural instability.


Permissible exposure limit. (OSHA)

personal protective equipment (PPE)

Respirators, gloves, eye protection, and other equipment used to protect workers from hazards when engineering controls fail to completely eliminate the potential for exposure.


Substance used to kill pests including: algae (algicide), aphids (aphicide), fungi (fungicide), plants (herbicide), insects (insecticide), larvae (larvacide), molluscs (molluscicide), eggs (ovicide), rodents (rodenticide), and slime-producing organisms (slimicide).

photoallergic contact dermatitis (PACD)

A type of allergic contact dermatitis caused by skin contact with an allergen that becomes active only after it absorbs UV light

photoirritant contact dermatitis (PICD)

A type of irritant contact dermatitis caused by skin contact with an irritant that becomes active only after it absorbs UV light.


A thin membrane that covers the lungs (visceral pleura) and thorax adjacent to the lungs (parietal pleura).


Chronic scarring lung disease caused by the accumulation of asbestos, coal, silica, and other fibrogenic dusts.


Inflammation of the lungs induced by inhalation of metal fumes or toxic gases and vapors.

positron decay

         "A positron is a particle that has the same mass as an electron but has a positive charge. Positron decay may occur when there are too many protons in the nucleus. In this case, a proton is converted into a neutron, and nucleus emits a positron. This increases the number of neutrons by one, decreases the number of protons by one, and leaves the atomic mass unchanged. By changing the numbers of protons, however, positron decay transforms the nuclide into a different element." EPA website: Understanding Radiation


Parts per million, e.g., an air concentration of benzene of 80 ppm is 80 parts of benzene per million parts of air;


Pertaining to the lung.

Radiation Accident

Large-scale accidents from atomic bomb testing fallout released iodine-131 and strontium-90. The nuclear accident at Chernobyl released large amounts of radioactive iodine and cesium. Small-scale accidents usually occur as the result of exposures to "industrial radiography sources used in nondestructive testing, in industrial sterilization facilities, in radiotherapy machines, as well as from criticality accidents occurring with experimental assemblies or chemical processing." [Gusev, p. 17-19; also see Table 7-1: Major Radiation Accidents (1945 to 2000) and Table 7.2: Meltings of Radioactive Materials]

Radiation Energy

The usual energy ranges for the common forms of radiation are 4 to 8 MeV for alpha particles, 0 to 2 MeV for beta particles, and 0.05 to 2 MeV for gamma rays. ORAU Radiation Basics See Appendix A-1 in Gollnick for a list of 62 radionuclides with their major radiations and energies. The list shows the percentage of disintegrations which produce the radiation and the maximum of the spectrum for beta and positron sources.


"having the property of emitting radiation (such as alpha, beta, or gamma rays) from an atomic nucleus" NTP Glossary


A radioisotope, which is defined as "an unstable isotope of an element that decays or disintegrates spontaneously, emitting radiation. Approximately 5,000 natural and artificial radioisotopes have been identified." NRC Glossary

Radionuclide Internal Toxicity Class

Based on the annual limit on intake (ALI): >5 mCi is Low; 0.500-4.999 mCi is Moderate; 0.005-0.499 mCi is High; <0.005 mCi is Very High; Vanderbilt University Online Radiation Safety


Concentration producing a 50% decrease in respiratory rate in experimental animals following a 10-minute exposure.

Reference Dose (RfD)

"The RfD is a numerical estimate of a daily oral exposure to the human population, including sensitive subgroups such as children, that is not likely to cause harmful effects during a lifetime." [EPA Glossary] See "ADI."


Recommended exposure limit. (NIOSH)


Pertaining to the kidneys.

reproductive toxin

A chemical that is toxic to the reproductive system, including defects in the progeny and injury to male or female reproductive function. Reproductive toxicity includes developmental effects. See Guidelines for Reproductive Toxicity Risk Assessment.

Restricted Use

Regulations that have banned or restricted the use of the agent.

route of exposure

Route of entry; For occupational exposures, poisons are usually absorbed through inhalation or skin contact. Significant occupational lead absorption can occur through ingestion.


An agent that can induce an allergic reaction in the skin or lungs.


"Watery proteinaceous portion of the blood that remains after clotting." [IUPAC Gold Book]


Objective evidence of disease from a laboratory test, x-ray, or physical examination.

simple asphyxiant

A gas or vapor that can cause asphyxiation by displacing oxygen, but has no other significant adverse effects at low concentrations. "In general, simple asphyxiants have no pharmacologic activity." [Goldfrank, p. 1643] Carbon dioxide causes tachypnea at concentrations >2-3% and death within minutes at concentrations >10%. [LaDou, p. 518] Deep-sea divers may suffer from inert gas narcosis (xenon) and nitrogen narcosis. [Wald, p. 171] Volatile substances of abuse include aliphatic hydrocarbons such as methane, ethane, acetylene, and butane. [Ford, p. 651]

skin designation

"Danger of cutaneous absorption." (ACGIH)


"an establishment used to separate or refine a metal from its ore, often with accompanying chemical change." NTP Glossary


A simple rule is that a substance is soluble in water when >1 g/100 ml (>10 g/L) dissolves and insoluble when <0.1 g/100ml (<1 g/L) dissolves. For very toxic substances pay attention to the quantity (the dose) that dissolves. Soluble metal compounds include nitrates, acetates, chlorides, bromides, and iodides (except those of Ag, Hg, and Pb); Also soluble are sulfates (except Ba, Sr, and Pb) and all salts of Na, K, and NH4 (except Na.1/3O4Sb, Cl6-Pt.2K, and potassium cobaltnitrite).  [Nordberg, p. 15] 

Specific Activity

Disintegrations per gram of a radioisotope. Equals 1 Ci/gm for radium-226. "Note that the activity, alone, is not a direct measure of the hazard of a radioactive sample. The relative hazard also depends on the types of radiation emitted in the decays and on the number of such emissions per decay. As an example, working 1 meter away from a 4 terabecquerel (108 Ci) source of cobalt-60 for 8 hours would deliver a whole body dose of about 11.3 Sv (1130 rem) to a person--a lethal dose. Work performed under the same conditions with a 4 TBq (108 Ci) source of hydrogen-3 would produce no effect at all. The low 0.018 MeV maximum beta would be totally absorbed by the air between the worker and the source." [Gollnick, p. 123]


Short-term public emergency guidance level. (NRC)


Short-term exposure limits. (ACGIH)

subacute syndrome

The onset of symptoms is gradual over a period of less than 2 months. The syndrome may be a cumulative exposure with short latency, e.g., lead poisoning.


"A detergent compound that promotes lathering." [EPA]

Tenth-Value Layer (TVL)

"The thickness of a specified substance which, when introduced into the path of a given beam of radiation, reduces the radiation field quantity to one-tenth of its original value." NCRP Radiological Terms For values, see Vanderbilt University Online Radiation Safety.


"A substance capable of causing birth defects." [EPA]

Threshold Limit Value (TLV)

"The concentration in air to which it is believed that most workers can be exposed daily without an adverse effect (i.e., effectively, the threshold between safe and dangerous concentrations). The values were established (and are revised annually) by the ACGIH and are time-weighted concentrations (TWA) for a 7- or 8-h workday and 40-h workweek, and thus are related to chronic effects. A short-term exposure limit (STEL) is defined as a 15-min TWA exposure, which should not be exceeded at any time during a workday even if the 8-h TWA is within the TLV-TWA." IUPAC Gold Book

TIH (Toxic Inhalation Hazard)

"Term used to describe gases and volatile liquids that are toxic when inhaled." Some are TIH materials themselves, e.g., chlorine, and some release TIH gases when spilled in water, e.g., chlorosilanes. [ERG 2016]


Threshold limit value. (ACGIH)


"The amount of pesticide that may safely remain in or on raw farm products at time of sale." [EPA]


Time-Weighted Average, the concentration of a chemical averaged or weighted over an 8-hour workday.


A chemical like pentachlorophenol that can cause a hypermetabolic state by poisoning cellular respiration (uncoupling oxidative phosphorylation).

Vapor Pressure (VP)

A measure of a chemical's volatility at room temperature (20-25° C or 68-77° F). Multiply vapor pressure times 1300 to estimate in ppm the saturated concentration of the chemical after a spill in a confined space. [Sullivan, p. 34]


Quality of a solid or liquid allowing it to pass into a vapor state at a given temperature. [NTP]

Water Reactive Material

A material that "produces significant toxic gas when it comes in contact with water." [ERG 2016]


Workplace Environmental Exposure Levels. [AIHA]

To convert between ppm and mg/m3, see the Conversion Calculator

Two places to look for molecular weights are HSDB and the NIOSH Pocket Guide.

Molecular Weight Calculator