Page 451 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

432 14 Indoor Air Quality

than the summation of all individual effects. In order to quantify the combined
effect, normalized concentration, C N , should be used [2]. Mathematically, it is,

C i
X
C N ¼ ð14:1Þ
TLV i
where C i and TLV i are the concentration and threshold limit value of the ith air
pollutant of concern, respectively. The equation shows that the normalized con-
taminant concentration is a dimensionless parameter as long as the units in the
nominator and denominator match for each air pollutant counted.
When C N is less than unity, the air quality is considered acceptable. Otherwise,
the air quality needs improvement by reducing the concentrations of one or multiple
air pollutants. In reality, however, it is very challenging to enforce the normalized
concentration, because there is always a chance that it is greater than unity, pro-
vided the list of the pollutants is long enough.
Example 14.1: Threshold limit value
In a welding shop, the measured concentrations of CO, CO 2 and welding fumes are
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10, 1,500 ppmv and 3.5 mg/m , respectively, all below the recommended TLV-
TWA. Is this working environment safe to the workers daily based on normalized
concentration?
Solution
TLV-TWA can be found in Table 14.2. And they are listed as follows

Table 14.2 Threshold values of typical indoor air pollutants in work places
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Particulate pollutant TWA (mg/m , except for asbestos)
Asbestos 0.1 fiber/ml
Coal dust, anthracite 0.4
Coal dust, bituminous 0.9
Grain dust (Oat, wheat, barley) 4.0
Graphite (non fiber) 2.0
Iron oxide particles and fume, inhalable 5.0
Lead 0.05
Welding fumes 5.0
Gases Formula TWA (ppmv) STEL (ppmv)
Ammonia NH 3 25 35
Carbon dioxide CO 2 5,000 30,000
Carbon monoxide CO 25
Formaldehyde HCHO – 0.3
Hydrogen sulfide H 2 S 10 15
Methanol CH 3 OH 200 250
Ozone O 3 0.05–0.2 –
Sources ACGIH [11], WHO [7]