calculations

CALCULATIONS

·        If the initial and final calibration flow rates are different, a volume calculated using the highest flow rate should be reported to the laboratory. If compliance is not established using the lowest flow rate, further sampling should be considered.

·        Generally, sampling is conducted at approximately the same temperature and pressure as calibration, in which case no correction for temperature and pressure is required and the sample volume reported to the laboratory is the volume actually measured. Where sampling is conducted at a substantially different temperature or pressure than calibration, an adjustment to the measured air volume may be required depending on the sampling pump used, in order to obtain the actual air volume sampled.

·        The actual volume of air sampled at the sampling site is reported, and used in all calculations.

The laboratory normally does not measure concentrations of gases and vapors directly in parts per million (ppm). Rather, most analytical techniques determine the total weight of contaminant in collection medium. The lab calculates concentration in mg/m3 and converts this to ppm at 25 ⁰C and 760 mm Hg using following Equation. This result is to be compared with the PEL without adjustment for temperature and pressure at the sampling site.

TLV in ppm =  (TLV in mg/m³  ) X (24.45) / (Gram Molecular weight of

                                                                             substance)

OR

TLV in mg/m³ = (TLV in ppm) X (Gram Molecular weight of substance)/24.45

Where:

24.45 =molar volume at 25  C(298 K) and 760 mm Hg

Mwt =molecular weight

NTP =Normal Temperature and Pressure at 25 C and 760 mm Hg

NOTE: When a laboratory result is reported as mg/m3 contaminant, concentrations expressed as ppm (PT) cannot be compared directly to the standards table without converting to NTP.

Adjustment for temperature and pressure: Formula is as bellow:

ppm(PT) = mg/m3 X 24.45/ Mwt X 760/P  X  298/T

·        Time-Weighted Average: The average full shift exposure level calculated by weighing the various concentrations throughout the workday with respect to time.

TWA = C₁T₁ + C₂T₂ + CnTn  / 8 hr

            Where, TWA = Time-weighted average concentrations in ppm/ or mg/m³

                                 C        = Concentration of contaminant during an incremental exposure  

                                        time

                          T       = Time : Incremental Exposure Time

·        Threshold Limit Values for Mixtures :

Ø    Most threshold limit values are developed for a single chemical substance.

Ø    However, the work environment is often composed of multiple chemical exposures both simultaneously and sequentially.

Ø    It is recommended that multiple exposures that comprise such work environments be examined to assure that workers do not experience harmful effects.

Ø    There are several possible modes of chemical mixture interaction.

1)    Additivity occurs when the combined biological effect of the component is equal to the sum of each of the agents given alone.

2)    Synergy occurs where the combined effect is greater than the sum of each agent. 

3)    Antagonism (Reveres Effect) occurs when the combined effect is less.

Ø    The general ACGIH mixture formula applies to the additive model.

NOTE: The guidance contained does not apply to substances in mixed phases.

Ø When two or more hazardous substances have a similar toxicological effect on the same target organ or system, their combined effect, rather than that of either individually, should be given primary consideration. In the absence of information to the contrary, different substances should be considered as additive where the health effect and target organ or system is the same.

That is, if the sum of:  C₁/T₁ + C₂/T₂ +…..Cn /Tn = 1

Where, C₁ indicates the observed atmospheric concentration and T₁ is the corresponding threshold limit)

It is essential that the atmosphere is analyzed both qualitatively and quantitatively for each component present in order to evaluate the threshold limit of the mixture.

Example: Air contains 400 ppm of acetone ( TLV,750 ppm),150 ppm of sec-butyl acetate (TLV,200 ppm) and 100 ppm of methyl ethyl ketone (TLV,200 ppm).

400/750 + 150/200 + 100/200 = 0.53 + 0.75 + 0.5 = 1.78

Threshold limit is exceeded.

Ø Special case when the source of contaminant is a liquid mixture and the atmospheric composition is assumed to be similar to that of the original material, e.g. on a time-weighted average exposure basis, all of the liquid (Solvent) mixture eventually evaporates. When the percent composition ( by weight) of the liquid mixture is known, the TLV s of the constituents must be listed mg/m3.

1/ fa/TLVa+fb/TLVb+fc/TLVc +…fn/TLVn

Example: Liquid contains ( By Weight)

          50% heptan e: TLV 400 ppm or 1600 mg/m3

          30% methyl chloroform : TLV 350 ppm or 1900 mg/m3

          20% perchloroethylene : TLV 50 ppm or 335 mg/m3

          TLV of Mixture = 1/0.5/1600+ 0.3/1900 + 0.2/335

                                   =  1/0.00031 + 0.00016 + 0.0006

                                   =  1/0.00107 =  935 mg/m3

of this mixture:

50% heptane or (935)(0.5) = 468 mg/m3 is heptane

30% methyl chloroform (935)(0.3) =  281 mg/m3 is methyl chloroform

20% perchloroethylen (935)(0.2) = 187 mg/m3 is perchloroethylen

These values can be converted to ppm :

Heptane = 117 ppm

methyl chloroform = 51 ppm

perchloroethylen = 29 ppm

TLV of mixture =  117 + 51 + 29 = 197 ppm