Here each risk is expressed as a unitless decimal (or scientific notation) and we sum the risks. The pathway of the risk is indicate by the subscripts.

Risk _{dermal at site} + Risk _{ingestion at site} + Risk _{ingestion at home} + Risk _{inhalation at home} = Total Cancer Risk

The assumption is that, at the DNA level where the cancer causing mutations are, it does not matter by what route the chemicals entered the receptor. In the write up of your calculations, you want to keep the risk for each route separate, because later when the risk management decisions are made, you may only need to eliminate one route in order to bring the risk into acceptable levels.

Now suppose you had two chemicals at the site, PCP and chemical Z. When computing the total risk, you just add the risks due to chemical Z to those of PCP:

Risk _{PCP,dermal at site} + Risk _{PCP, ingestion at site} +Risk _{PCP, ingestion at home}+ Risk _{PCP, inhalation at home} +Risk _{Z, dermal at site} + Risk _{Z, ingestion at site} +Risk _{Z, ingestion at home}+ Risk _{Z, inhalation at home} = Total Risk

Unlike the example above where both PCP and Z have the same exposure routes, it is possible that the routes of Z are different. For example if Z was volatile as well, you might have Risk _{Z, inhalation of vapors at site} which you would just add with the others. Certain chemicals cause cancer at certain sites, therefore the addition of the risk of chemicals may be conservative. It is certainly a lot easier, and is the standard assumption. If you did not want to add them, you would need a toxicologist to report that the chemical causes different types of cancer only at certain organs and that these are different as well.

Generally you summarize your risk calculations in a table that may look like this:

Current Situation (Baseline)
Receptor	Pathway	Cancer Risk
Kids		PCP	Z
	Ingestion at home	5 x 10 -6	2 x 10 -8
	Inhalation at home	2 x 10 -6	1 x 10 -7
	Ingestion at site	2 x 10 -7	2 x 10 -7
	Direct dermal at site	4 x 10 -7	1 x 10 -7
	Inhalation of vapors at site	N/A	2 x 10 -5
	Total Risk per Chemical	8 x 10 -6	2 x 10 -5
	Total Risk	3 x 10 -5

Note several things about the table.

• The risk is expressed to one significant figure. In your earlier calculations you may have carried as many significant figures as you felt were justified, but it is unlikely that any of the parameters were accurate to more than plus or minus 10%. For this presentation you multiplied all these inaccuracies together and it is unlikely your final number is accurate to more than one figure.
• In the risk for chemical Z, there was no point in adding the numbers. The risk for inhalation was 100-fold larger than the next highest risk, ingestion at the site, so that would contribute no more than 1% to the final answer, and will not effect the significant figures of the answer. You often find this when adding numbers in scientific notation.
• The "N/A" means "not applicable" sometime a dash "-" is inserted instead. Either indicates to the reader that the empty cell in the table is not a mistake. N/A is preferred to placing a zero "0" in the table, since philosophically at least, there is always some small risk.

In your report you would probably extend the table down for the other receptors in this current or baseline situation, or you might make a new table for each receptor. If you had 5 or 6 chemicals you might extend the table to the right, adding columns for each chemicals. Also, you may want to highlight or bold the risks that are above the acceptable level. Uh oh, now I got to tell you about acceptable risks. NEXT

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