Conversions,
Calculations and Concentrations

** **

**CONVERSIONS**

1000 ng (nanograms) = 1.0 ug
(microgram)

1000 ug (micrograms) = 1.0 mg (milligram)

1000 mg
(milligrams) =
1.0 g (gram)

1000 g
(grams) = 1.0
Kg (kilogram)

1000 uL (microliters) = 1.0 mL
(milliliter)

1000 mL (milliliters) = 1.0 L (liter)

1000 L
(liters) = 1.0 M^{3 }(cubic
meter)

1.0 mL (milliliter) = 1.0 cc (cubic centimeter)

Some
samples of these conversions are:

7465 ng = 7.465 ug 7.465 ug =
0.007465 mg

0.339 mg
= 339 ug
0.7891 Kg = 789.1 g

244.7 mL = 0.2447 L 1.349
M^{3} = 1349 L

1428.7 L
= 1.4287 M^{3 }546 cc = 546 mL

**CALCULATING AIR VOLUMES **

Calculating accurate air volumes (exact
amounts of air drawn though air sampling media) is imperative in
reaching meaningful results. The air
volume is simply the product of two terms, flowrate
and sampling time:

Air Volume = Flowrate x
Sampling Time

The flowrate is
usually expressed in liters/minute or in milliliters/minute (mL/minute is the same as cc/minute). The sampling time needs to be expressed in
minutes, and therefore the product of these two terms, the air volume, will be
either liters or milliliters.

Some
samples of calculating air volumes are:

11.7 L/ min. x 240 minutes = 2808 L

20 mL/min. x 462
minutes = 9240 mL (= 9.240 L)

540 cc/min. x 366 minutes = 197640 cc (= 197.640 L)

3.4 L/min. x 480 minutes = 1632 L (= 1.632 M^{3})

**CONCENTRATIONS :**** AN IMPORTANT CONCEPT **

Concentration
is an important concept in expressing analytical results in environmental and
industrial hygiene testing. A
concentration is so much of something __per__ something else. When dealing with concentrations, the word __per__
is used to separate two quantities, such as in “parts per million” and mg/M^{3 }. The
most commonly used concentrations are:

mg/Kg (milligrams per kilogram)

mg/M^{3} (milligrams per cubic meter)

PPM (parts per million)

fibers/cc (fibers per cubic centimeter of air)

mg/L (milligrams per liter)

Note
that there are basically two types of concentrations, weight per weight and
weight per volume. Weight per weight
concentrations (mg/Kg) are used to express contaminant levels of solids, such
as the level of lead in paint chips.
Weight per volume concentrations (mg/M^{3} and mg/L) are used to
express contaminant levels of both air and liquids. The amount of benzene vapor in air and the
amount of iron in water (most likely expressed as mg/M^{3} and mg/L,
respectively) would both be weight per volume concentrations.

Obviously,
concentrations are necessary to make sense out of analytical results. Knowing only the total weight of metal found
on a filter is useless without knowing the exact amount of air in which that
amount of metal was contained.

**WHEN SENDING SAMPLES TO SCHNEIDER LABORATORIES,
INC. PLEASE INCLUDE THE AIR VOLUMES IF THEY ARE AIR SAMPLES.**** THE CONCENTRATION CANNOT BE CALCULATED
WITHOUT THE AIR VOLUME. **

** **

The
concentration can always be determined if the amount of contaminant and the
volume of air drawn through the sampler are known. Start with the weight amount of contaminant
found on the entire sample, then divide by the air volume to arrive at the
concentration, for example:

A
charcoal tube is sent to the lab for benzene analysis, but the volume of air,
24.17 L, was not included. The lab
produces a result of 0.03 mg benzene.
To compare these results to a PEL or TLV, the concentration is needed.

__0.03 mg benzene =__ 1.24 mg/M^{3}
benzene

0.02417 M^{3} air

Notice
that liters were converted to cubic meters so that the result may be expressed
as mg/M^{3}, not mg/L.

For
fiber counting, as in asbestos analysis, the application is similar. The concentration is now __number of fibers__
per __volume of air__.

A
1243.7 L air sample is sent to the lab, but the air volume is not
included. A result of 7847
fibers/filter was reported. Knowing the
total number of fibers on the entire filter and the air volume, the
concentration of fibers in air can easily be calculated:

7847 __fibers__ = 0.0063 fibers/cc

1,243,000 cc air

Notice
the converstion from liters to cubic centimeters, so
that the results are in fibers/cc not in fibers/L.

Remember,
when working with concentrations, both the weight value and the volume value
can be changed into the preferred units before the division is done. For example, a result of

517.3 mg/L chlordane in water

can be correctly expressed as any of the following, without
changing the result:

517300 ug/L 517.3 ug/mL

One
last word about concentrations: Parts
per million, or PPM, is frequently used to express concentrations such as
volume per volume of air, volume per volume of liquid (usually water - aqueous
solutions), and weight per weight.
For example, in testing a solid for lead we found that it contained 34 ug of lead
per gram of solid, we could say that the solid
contained 34 PPM lead, since there are a million micrograms in one gram.

Applying
PPM values to lead contamination in water is not difficult if we remember one
very important premise in chemistry.
One milliliter (1.0 mL) of water weighs
exactly one gram (1.0g). Knowing this,
it is easy to understand that if water contains 69 ug
of lead per mL, then that water is 69 PPM lead. This is because 69 ug/mL is the equivalent of saying 69 ug/g (remember, 1.0 mL of water
weighs 1.0 g). So, 69
ug of lead per gram of water means, by definition, 69
PPM.

Lastly,
PPM can refer to volume of contaminant per volume of air. In this case the contaminant is usually an
organic vapor. This type of PPM
application is the most difficult to understand. It is based on the following two chemical
principles:

1)
A
chemical’s molecular weight
is the number of grams of that chemical found in one mole of that
chemical.

2)
At
25^{o }C temperature and 1 atmosphere pressure, one mole of any
chemical (in a vapor state if it is normally a liquid) or gas occupies exactly
the same amount of air space, 24.45 liters.
Volume per air volume results of solvent vapors and gases in PPM, can be
expressed by using the following:

__mg____
contaminant__ x __24.45 L/mole__ =_{
}PPM contaminant

M^{3} air m.w. as g/mole in air_{ }

Always
remember, when using PPM, volume or weight per volume must always be expressed
- never use “mixed” values such as weight per volume.

Sometimes
it is useful to convert results expressed in PPM to results expressed as a
percentage (or vice versa), as shown below:

parts per million =
(percent) x (0.0001)

Percent = __parts per million __

0.0001

** **

** **

**TIME-WEIGHTED AVERAGES **

** **

Time
weighted averages are expressions on concentrations that have been “time
weighted” for the purpose of comparison with 8-hour contaminant
concentrations. In other words, if a
sample result (always as a concentration) is multiplied by a factor that would
make the resultant concentration represented the contaminant level __if the
sampling time had been exactly eight hours__, instead of more or less than
eight hours. If sampled for exactly 8
hours, the sample result (concentration) is already, by definition, an 8-hour
TWA.

If
the sampling is __less__ than eight hours, then the sample concentration is
multiplied by a factor that is __less than 1__. This would take the concentration of
contaminant to which the employee was exposed and “spread it out” to represent
what the concentration would have been if the exposure was 8 hours. The __weight__ of the contaminant would
be unchanged. If sampling is __more__
than 8 hours (for instance a worker who works 4 10-hour days), the sample
concentration would be multiplied by a factor that is __greater than 1__;
this would take the concentration of contaminant to which the employee was
exposed and “squeeze it into an 8-hour time frame” to represent what the
concentration would have been if the exposure was 8 hours, but the __weight__
of the contaminant would be unchanged.

TWA’s are expressed as either PPM or mg/M^{3}
values. Hygienists need to calculate TWA’s for all their sample results because the OSHA PEL’s and the ACGIH TLV’s are all
expressed as either PPM TWA’s or as mg/M^{3} TWA’s. In order to
compare results to the standards, both values need to be time weighted
averages. The following calculations
are used to arrive at 8-hour TWA’s.

__mg__ x __minutes sampled__ = __mg__ TWA

M^{3} 480 minutes M^{3 }

^{ }PPM x_{ }__minutes sampled__ = PPM TWA

480 minutes

While
sampling a painter for mineral spirits, the pump ran for 6 hours and 15 minutes
while he was painting, and he spent the remainder of his shift unexposed to
solvent vapors. The lab reports a
result of 0.344 mg/M^{3} mineral spirits for the charcoal tube. The following converts this concentration to
an 8-hour TWA.

0.344 mg/M^{3} x __375 minutes__ = 0.269 mg/M^{3} TWA

480 minutes

Remember,
the painter was sampled for less than 8 hours, so the factor multiplied by the
concentration is less than 1.

A
lab report for formaldehyde states that the exposure was 0.85 PPM. This sample was taken on an employee working
a 12-hour shift, but was sampled for 11 hours and 15 minutes because he took a
break. An 8-hour TWA is calculated in
order to compare his exposure to the standard.

0.85 PPM x __675 minutes =__ 1.20 PPM TWA

480 minutes

Since
sampling was more than 8 hours, a factor greater than 1 is multiplied raising
the concentration to represent what the concentration would have been during an
8-hour period had the __weight__ of formaldehyde found remained the
same.