Overview :

Chemical substances can have adverse effects on people, depending on the nature of the chemical, its concentration, the nature and duration of the exposure and individual susceptibility. A Threshold Limit Value (TLV) for a chemical is the maximum amount of airborne chemical which people may be exposed to repeatedly without causing adverse effect.

Different categories of TLVs exist:

(1) Ceiling: This is the amount of chemical that should not be exceeded at any time;

(2) Short Term Exposure Limit (STEL): This is the concentration of a compound which a person may be exposed to for a short period of time (15 minutes) without tissue damage, irritation or other adverse effect;

(3) Time Weighted Average (TWA): This is the concentration of chemical that a person can be repeatedly exposed to over a long period without adverse effect, based on a time-weighted average concentration for the average work week of 40 hours as five 8 hour days.

TLVs are based on population studies and may over- or under-estimate the risk to the individual, due to:

(1) hypersensitivity or tolerance to the chemical

(2) concurrent disease making a person more susceptible to drug toxicity

(3) interactions with other chemicals or drugs

(4) genetic variation in metabolic pathways

In addition, persons may have multiple exposures to a chemical at different times and locations, which may contribute to cumulative toxicity.

American Conference of Governmental Industrial Hygienists. 1994-1995 Threshold Limit Values for Chemical and Physical Agents and Biological Exposure Indices. 1994. pages 2-43

>>>33.01.01 Threshold Limit Values for Mixtures of Airborne Chemicals

Overview:

If a person is exposed to a mixture of chemicals, then the threshold limit value (TLV) for the mixture can be estimated by adding together the ratio for the concentration to the TLV for each chemical.

Threshold Limit Value for a chemical mixture of N compounds =

= sum of ratios for the concentration of each chemical to its TLV=

= ((concentration of chemical #1) / (TLV for chemical #1)) + ((concentration of chemical #2) / (TLV for chemical #2)) + ..... + ((concentration of chemical #N) / (TLV for chemical #N)) 

Interpretation:

• If the sum of the ratios <= 1, then the threshold is not exceeded.

• If the sum of the ratios > 1, then the threshold is exceeded.

Converting Units

Data on TLV is expressed as parts per million (ppm) or milligrams per cubic meter of air (mg/m^3).

TLV in mg/m^3 =

= (TLV in ppm) * (gram molecular weight of substance) / (24.5)

TLV in ppm =

= (TLV in mg/m^3) * (24.5) / (gram molecular weight of substance)

References:

American Conference of Governmental Industrial Hygienists. 1994-1995 Threshold Limit Values for Chemical and Physical Agents and Biological Exposure Indices. 1994. pages 2-43

Walker TJ. Chapter 2: Recognition and evaluation of hazards. pages 10-32 (31-32). IN: Proctor NH, Hughes JP. Chemical Hazards in the Workplace. JB Lippincott. 1978.