Transitions - Sustainable Design: The Science of Sustainability and Green Engineering - page 148

If the intent is to cut total VOC emissions, clearly the first target should
be the methyl chloride, at least in terms of the mass released. But another
important consideration in preventing pollution is relative risk .
Although methyl chloride is two orders of magnitude more volatile than the
other pollutants, all three compounds are likely to be found in the atmosphere.
Thus, inhalation is a likely exposure pathway.
Since risk is the product of exposure times hazard ( R H ), we can

compare the risks by applying a hazard value (e.g., cancer potency). We can use
the air emissions calculated above as a reasonable approximation of exposure
via the inhalation pathway, * and the inhalation cancer slope factors can be used
to represent the hazard. These slope factors are published by the U.S. EPA and
are found to be:

= E ×

053 kg daymg − 1 carbon tetrachloride : 0 .

0035 kg daymg − 1
trichloroethylene : 0

methyl chloride : 0 .

0063 kg daymg − 1 .

The relative cancer risk for the three compounds can be estimated by
removing the units (i.e., we are not actually calculating the risk, only comparing
the three compounds against each other, so we do not need units. If we were
calculating risks, the units for exposure would be mass of contaminant per
body mass per time (e.g., mg kg − 1 day − 1 ), whereas the slope factor unit is the
inverse of this (i.e., kg mg − 1 ), so risk itself is a unitless probability.

· day ·

carbon tetrachloride : 0 . 053 × 775 = 41

methyl chloride : 0 . 0035 × 16000 = 56

trichloroethylene : 0 . 0063 × 7800 = 49

Thus, in terms of relative risk, methyl chloride is again the most important
target chemical, but the other two are much closer. In fact, given the uncer-
tainties and assumptions, from a relative risk perspective, the importance of the
removing the three compounds is nearly identical, owing to the much higher
cancer potency of CCl 4 .

Source : D. A. Vallero and P. A. Vesilind, Socially Responsible Engineering: Justice
in Risk Management, Wiley, Hoboken, NJ, 2006.

* Even without calculating the releases, it is probably reasonable to assume that the exposures will
be similar since the three compounds have high vapor pressures (more likely to enter the vapor
phase and to be inhaled): carbon tetrachloride, 115 mm Hg; methyl chloride, 4300 mmHg; and
trichloroethylene, 69 mmHg.