thermometer

The weather may be getting colder in the Northern Hemisphere, but the mercury is rising. Mercury (Hg), toxic to most organisms, is present in the natural environment in small quantities. However, through industrial activities such as coal-burning power plants, humans cause the release of mercury into the atmosphere at concentrations higher than would naturally occur. The mercury then rains down with precipitation and is deposited on land and in water.

In water, microorganisms convert mercury to methylmercury, a more lethal compound. Through a process known as bioaccumulation methylmercury builds up in the body tissues of predators when they feed upon contaminated prey. Large predatory fish in lakes and rivers are susceptible to bioaccumulation of methylmercury as well as oceanic top predators such as shark, swordfish, king mackerel and tilefish. Current scientific research seems to indicate that smaller oceanic fish are not showing high levels of methylmercury.

Mercury is a neurotoxin, affecting the brain and nervous system, especially developing brains. For this reason, it is very important that pregnant and breastfeeding women and young children avoid any foods that may contain high levels of mercury. The U.S. Environmental Protection Agency (EPA) determined a reference dose (RfD) for methylmercury of 0.0001 mg per kg of body weight per day. Therefore, a person weighing 60 kg (or 132 lbs) should not consume more than 0.006 mg of methylmercury per day or 0.18 mg of methylmercury per month (0.006 x 30 days).

Since 1993, the EPA has issued guidelines on which fish are safe to eat based on the species of fish, its size (smaller fish will have less methylmercury accumulation), and the water body in which it lived. For reducing contaminants other than mercury, there are even guidelines for preparing fish such as trimming skin and fat and grilling to allow fat to drip away. Unfortunately, these methods do not work for mercury because it is stored throughout the fish's body not just its fat. From 2002 to 2003, the number of advisories related to mercury in fish increased from 2,100 in 44 states to 3,089 in 48 states. To find out if fish in your local water body have mercury contamination problems, check out the National Listing of Fish Advisories . Click on "Map Advisories", then select "Mercury" as your pollutant and your state.

Seafood is a very important low-fat, high protein food source and should not be eliminated from our diets. Instead, we need to be aware of which species of fish may contain high levels of methylmercury and then limit consumption of those fish to the RfD level.

DATA ACTIVITY

Based on the EPA's RfD for methylmercury, we will calculate the amount of fish a person can eat per month in order to stay within acceptable limits. Print copies of the blank worksheet.

Scenario: A family, consisting of a husband, wife and 8 year old daughter, have spent the day fishing on a nearby freshwater lake. After a successful day, they bring home several walleye and intend to eat them for dinner that night. As they are preparing the fish, the mother remembers an article in the paper warning of mercury in fish. According to the article, walleye from their lake have been averaging 1 parts per million (ppm) of methylmercury. The article also provides them with the EPA reference dose of 0.0001 mg per kg of body weight per day. Not wanting to ingest more than the acceptable level of methylmercury, the family sits down to see how much, if any, of the walleye each person can eat.

Based on the EPA advisory and known contamination level for walleye in the lake, determine how much of their day's catch the family can eat to avoid exceeding the EPA methylmercury guidelines. Assume the family has not eaten any fish in the last month.

(Note: 1 ppm methylmercury = 0.001 mg methylmercury/ g of fish)

First, let's determine how much methylmercury (MM) is safe for each of the family members to consume in a given month. Calculate the missing values in the following table.

Table 1.
Family Member

Weight
(lb)

Weight
(kg)
RfD
(mg/kg per day)
Daily Allowable
MM Intake
(mg/day)
Monthly (30 days) Allowable MM Intake
(mg/month)
Dad
180
 
0.0001 mg/kg per day
 
 
Mom
140
 
0.0001 mg/kg per day
 
 
Daughter
60
 
0.0001 mg/kg per day
 
 

Calculate the missing values in Table 2 to determine the allowable portion size of walleye for each family member.

Table 2.
Family Member
Monthly Allowable
MM from Table 1
Concentration of MM
in Walleye
Safe Amount of
Walleye to Eat
(g of fish)

Safe Amount of
Walleye to Eat
(oz of fish)

(1 oz = 28.4 g)

Dad   0.001 mg MM/ g of fish    
Mom   0.001 mg MM/ g of fish    
Daughter   0.001 mg MM/ g of fish    

Compare your answers to the Teacher's Answer Sheet.

EXTENSION

Using mean methylmercury levels of other seafood species (USFDA Table 3), have students calculate the allowable portion sizes for each family member.

For more related resources, visit the Bridge's Seafood and Pollution pages.

If you have questions about the Data Tip of the Month or have suggestions for a future data tip, contact Lisa Lawrence, Bridge Data Project Manager.


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