PSAT Reading Practice Test 16

Questions 1-9 refer to the following information.

Mercury in Fish Passage

Mercury is an unusual element; it is a metal but
is liquid at room temperature. It is also a neurotoxin
and a teratogen, as it causes nerve damage and birth
defects. Mercury can be found just about every-
05where; it is in soil, in air, in household items, and
even in our food. Everyday objects, such as thermometers,
light switches, and fluorescent lightbulbs,
contain mercury in its elemental form. Batteries
can also contain mercury, but they contain it in the
10form of the inorganic compound mercury chloride.
Mercury can also exist as an organic compound,
the most common of which is methylmercury.
While we can take steps to avoid both elemental
and inorganic mercury, it is much harder to avoid
15methylmercury.
Most of the mercury in the environment
comes from the emissions of coal-burning power
plants; coal contains small amounts of mercury,
which are released into the air when coal burns.
20The concentration of mercury in the air from
power plants is very low, so it is not immediately
dangerous. However, the mercury is then washed
out of the air by rainstorms and eventually ends up
in lakes and oceans.
25The mercury deposited in the water does not
instantaneously get absorbed by fish, as elemental
mercury does not easily diffuse through cell
membranes. However, methylmercury diffuses
into cells easily, and certain anaerobic bacteria
30in the water convert the elemental mercury to
methylmercury as a by-product of their metabolic
processes. Methylmercury released into the water
by the bacteria diffuses into small single-celled
organisms called plankton. Small shrimp and other
35small animals eat the plankton and absorb the
methylmercury in the plankton during digestion.
Small fish eat the shrimp and then larger fish eat the
smaller fish; each time an animal preys on another
animal, the predator absorbs the methylmercury.
40Because each animal excretes the methylmercury
much more slowly than it absorbs it, methylmercury
builds up in the animal over time and is passed on
to whatever animal eats it, resulting in a process
called bioaccumulation.
45As people became aware of the bioaccumulation
of mercury in fish, many reacted by eliminating
seafood from their diet. However, seafood contains
certain omega-3 fatty acids that are important
for good health. People who do not eat enough
50of these fatty acids, especially eicosapentaenoic
acid (EPA) and docosahexaenoic acid (DHA), are
more likely to have heart attacks than people who
have enough EPA and DHA in their diet. Because
fish and shellfish, along with some algae, are the
55only sources of these fatty acids, eliminating them
from our diet might have worse health effects than
consuming small amounts of mercury.
Scientists have studied the effects of mercury by
conducting tests on animals and by studying various
60human populations and recording the amount of
mercury in their blood. By determining the levels of
mercury consumption that cause any of the known
symptoms of mercury poisoning, they were able
to identify a safe level of mercury consumption.
65The current recommendation is for humans to
take in less than 0.1 microgram of mercury for
every kilogram of weight per day. This means that
a 70-kilogram person (about 155 pounds) could
safely consume 7 micrograms of mercury per day.
70Since haddock averages about 0.055 micrograms
of mercury per gram, that person could safely eat
127 grams (about 4.5 ounces) of haddock per day.
On the other hand, swordfish averages about 0.995
micrograms of mercury per gram of fish, so the
7570-kilogram person could safely eat only about 7
grams (about one-quarter of an ounce) of swordfish
per day.
Nutritionists recommend that, rather than elimi-
nate fish from our diet, we try to eat more of the
80low-mercury fish and less of the high-mercury fish.
Low-mercury species tend to be smaller omnivo-
rous fish while high-mercury species tend to be the
largest carnivorous fish. Awareness of the par-
ticulars of this problem, accompanied by mindful
85eating habits, will keep us on the best course for
healthy eating.