48 ENVIRONMENTAL ENGINEmG
background is 25 mrem per source per year. Average background is about 360 mrem
per year. How many fatal cancers attributable to ionizing radiation would result in the
United States each year if the entire population were exposed at the level of the EPA
standard? How many cancers may be attributed to background? If only 10% of the
cancers were fatal each year, what percentage of the annual cancer deaths in the United
States would be attributed to exposure to background radioactivity? (Note what “unit
risk” means in this problem.)
3.4 The allowed occupational dose for ionizing radiation is 1 rem per year. By
what factor does a worker exposed to this dose over a working lifetime increase her
risk of cancer?
3.5 Workers in a chemical plant producing molded polyvinyl chloride plastics
suffered from hemangioma, a form of liver cancer that is usually fatal. During the
20 years of the plant’s operation, 20 employees out of 350 - the total number of
employees at the plant during those years - developed hemangioma. Does working
in the plant present an excess cancer risk? Why? What assumptions need to be made?
3.6 Additional, previously unavailable data on hemangioma incidence indicate
that, among people who have never worked in the plastics industry, there are only 10
deaths per 100,000 persons per year from hemangioma. How does this change your
answer to Problem 3.5?
latent cancer fatalities
(LCF). EPA regards an acceptable annual risk from any single source to be
A copper smelter emits arsenic into the air, and the average concentration within a
2-mile radius of the smelter is 5.5 pg/m3. Is the risk from smelter arsenic emissions
acceptable to EPA?
3.8 In the community of Problem 3.7, approximately 25,000 people live within
a 2-mile radius of the smelter. Assuming that the residents live there throughout their
lifetimes, how many excess LCF would be expected per year in this population?
3.9 Using the data of Problems 3.7 and 3.8, estimate an acceptable workplace
standard for ambient airborne arsenic.
3.10 Plutonium has a physiological half-life of one year. The isotope Pu-239 has
a radiological half-life of 24,600 years, and the isotope Pu-238, a radiological half-life
of 87 years. If a volunteer eats 5.0 mg of Pu-239, how much is left in his body after
three years? If he eats 5.0 mg of Pu-238, how much is left in his body after three
years?
3.11 During WWII, a U.S. pilot flying a single bombing mission over Europe
faced a 4% probability of being shot down and not returning. After successfully com-
pleting (and returning from) 25 missions, a pilot was not required to fly any more
missions. What was the probability that a pilot would be shot down on his 25th mission?
(The answer is not 100%!)
3.12 Can the event tree of Fig. 3-1 be drawn in different ways? Draw this event
tree in a different way, and estimate the probabilities in each branch.
3.13 It is widely assumed that the Love Canal and Times Beach incidents, in which
the resident population was unwittingly exposed to toxic chemicals in the environment
over a period of years, resulted in adverse health effects to that population. Design an
epidemiological study that would determine whether there actually were adverse health
3.7 The unit lifetime risk from airborne arsenic is 9.2 x