Threshold Limit Value
what is TLV ?
The concentration to which it is believed that a person can be exposed continuously for short period of time without suffering from irritation chronic or irreversible tissue damage and reduce work efficiency.
TLV are typically set for an 8-hour workday and a 40-hour workweek to ensure worker safety by limiting exposure to hazardous substances in the air.
TLV is a reserved term of the ACGIH (American Conference OF Governmental Industrial Hygienists)
TLV Is estimate based on the toxicity in human or animal of given chemical substances and the reliability of the latest sampling and analytical method.
The TLV is recommendation with only a guideline status.
TLV for chemical substances is defined as a concentration in air, typically for inhalation or skin exposure.
TLV unit are ppm (parts per million) of air for gases and mg/m3 (milligrams per cubic meter) for particulates such as dust, smoke and mist.
TLV mentioned in legal part schedule -2, factories act-1948.
TLV Formula :
This formula is not applicable to airborne particles.
24.45 = molar volume of air in liters at 25 degree Celsius and 760 torr.
This formula is for gases and vapours :
TLV in ppm = (mg/m^3) * 24.45 / gram molecular weight
TLV In mg/m^3 = TLV in ppm * gram molecular weight / 24.45
Three Categories Of TLV are specified :
- Time weighted average (TWA)
- Short term exposure limit (STEL)
- Celling (C)
1.) Threshold Limit Value – Time Weighted Average (TLV-TWA) : The TWA concentration for a conventional 8 hour workday and 40 hour per week worker may be repeatedly exposed, day after day for working lifetime without adverse effect.
TWA Formula and calculation :
C1 * T1 + C2 * T2 + C3 * T3…….+ Cn * Tn / 8
C = Concentration in ppm of substances
T = Time of period in hours for the substances
Threshold limit value calculation :
The employees is exposed to NH3 concentration of 40 ppm for first 2 hours in the shift and the remaining every hour he exposed to 25 ppm. calculate the TLV/TWA whether the TWA is exceeded.
TWA = C1 * T1 + C2 * T2 /8
= 40 * 2 + 25 * 6 / 8
= 80 + 150 /8
= 230 /8
= 28.74 ppm
NH3 TLV is 25 ppm so 28.74 ppm is exceeded
2.) TLV – STEL (short term exposure limit) : The maximum concentration chemical to which worker may be exposed continuously for up to 15 min without danger to health or work efficiency and safety.
Spot exposure for a duration of 15min that can not be repeated more than 4 times per day and there should be at least 60 minutes between successive exposure in the range.
3.) TLV – Celling (TLV-C) : Absolute concentration that should be exceeded during any part of the working exposure.
There are TLV for physical agent as well as chemical substances :
Physical agent ; Noise, Vibration, Radiation
Chemical agent : Chlorine, Ammonia, CS2, Solvents, Bromine
Method of define TLV :
- Research & engineering studies
- Animal studies
- Human studies
- Pathological surveys
Uses or Need of TLV :
- To know exposure level
- to know evaluate control measures and their effectively
- to asses health status of worker
- For research new TLV value
Limitation of TLV :
- This can not used for medico legal purpose.
- This is for use by industry only for technical purpose for evaluation of exposure at work place.
- personal habits like smoking, alcohol not safe level as TLV may be reduce.
- TLV tested in American standard of working (40 hour per week) and weather condition and with >50 kg test model, Indian working condition and Indian working people (48 hour per week0 for TLV refer factories act 1948,schedule 2.
- Limited experience data.
- personal susceptibility.
- set of working conditions.
- pre exiting illness.
- It refer to safe value not to dangerous.
Example :
| No. | Substance | TWA (ppm) | TWA ( mg/m^3) | STEL (15min0
(ppm) |
STEL
( mg/m^3) |
| 01 | Acetone | 750 | 1780 | 1000 | 2375 |
| 02 | Ammonia | 25 | 18 | – | – |
| 03 | Benzene | 10 | 30 | – | – |
| 04 | Bromine | 0.1 | 0.7 | 0.3 | 2.0 |
| 05 | CS2 | 10 | 30 | – | – |
| 06 | carbon monoxide | 50 | 55 | 400 | 440 |
| 07 | CCL4 | 5 | 30 | – | – |
| 08 | Chlorine | 1 | 3 | 3 | 9 |
| 09 | Chlorobenzene | 75 | 350 | – | – |
| 10 | Chloroform | 10 | 50 | – | – |
| 11 | Cyanides | – | 5 | – | – |
| 12 | Florine | 1 | 2 | 2 | 4 |
| 13 | H2S | 10 | 14 | 15 | 21 |
| 14 | HCL-C | 5 | 7 | – | – |
| 15 | Nitric Acid | 2 | 5 | 4 | 10 |
| 16 | Ozone | 0.1 | 0.2 | 0.3 | 0.6 |
| 17 | Phosgene | 0.1 | 0.4 | – | – |
| 18 | Pyridine | 5 | 15 | – | – |
| 19 | Toluene | 100 | 375 | 150 | 560 |
| 20 | Xylene | 100 | 435 | 150 | 655 |
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Q&A :
01) What Is TLV Full Form ?
Ans. Threshold Limit Value
02) What Is ACGIH Full Form ?
Ans. American Conference OF Governmental Industrial Hygienists
03) What is TLV Formula ?
Ans. TLV in ppm = (mg/m^3) * 24.45 / gram molecular weight
TLV In mg/m^3 = TLV in ppm * gram molecular weight / 24.4
04) What is ammonia TLV ?
Ans. 25 ppm and 18 mg/m^3
05) What is STEL Full Form ?
Ans. Short-Term Exposure Limit
