Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 401

Step 5: Obtain relative risk index (RRI) for various categories such as global
warming, ozone depletion, smog formation, acid rain, and carcino-
genicity.
Step 6: Adjust parameters: (i) go to Step 2 if risks are large and repeat calcula-
tions; or (ii) go to Step 1 if reactant substitutions are possible and repeat
calculations.

The RRI for a particular impact category is the sum of contributions for all chemicals
from a process weighted by their emission rates (Allen and Shonnard, 2002):

N

I category = ( dimensionless risk index for the category ) i × m i , (6.281)

i = 1

where i 1, ... , N is the number of compounds (species) involved. The risk indexes

are generally calculated for the following five categories: global warming poten-
tial (GWP), smog formation potential (SFP), acid rain potential (ARP), inhalation
toxicology potential (INHTP), and ingestion toxicology potential (INGTP). The first
three potentials are available from various resources for specific chemicals and are
summarized in Appendix D in Allen and Shonnard (2002). The last two (noncarcino-
genicity toxicity indices) are obtained from the reference dose (RfD) and reference
concentration(RfC)foringestion(oral)andinhalation,respectively.Similarly,thecar-
cinogenicity indices for ingestion (oral) or inhalation pathways are also defined using
the slope factors instead of the reference doses (see also Chapter 1 for descriptions
of these terms).
To obtain the INHTP or INGTP, one has to first estimate the concentration of a
chemical in air or water environments using the fugacity model. For examples, let
us assume we know the concentration of the chemical in water and air. Once this is
obtained, we can obtain the risk index as follows:

=

(C ia / RfC i )
(C bm,a / RfC bm ) ,

INHTP i =

(6.282)

where bm stands for a benchmark chemical.

(C iw / RfC i )
(C bm,w / RfC bm ) .

INGTP i = (6.283)

The carcinogenic potentials (both inhalation and ingestion) are determined by
substituting RfC and RfD with cancer SFs (see Chapter 1 for definition).
To illustrate the above methodology, an example is given below.

E XAMPLE 6.37 I LLUSTRATION OF E NVIRONMENTAL R ISK E VALUATION FOR A
C HEMICAL M ANUFACTURING P ROCESS

Let us consider a plastics manufacturing process that has to treat and recover materials
from a gaseous stream.The gaseous stream consists of benzene, toluene, and byproducts
continued