Specialty Testing Guide › Heavy Metals Testing
Fish is the main way most people take in mercury, as a form called methylmercury. For most adults, the risk depends on which fish you eat and how often, not on whether you eat fish at all. A few large predatory species carry much more mercury than common choices like salmon, shrimp or canned light tuna.
If you fish in Pennsylvania, there is a second layer: state advisories on how often to eat what you catch. Pregnancy and young children call for the most care, and also for continuing to eat the right fish.
Key takeaways
- Most mercury in people comes from fish, and a handful of species carry far more than the rest.
- Federal advice for pregnancy is to keep eating fish, choosing two to three servings a week from lower-mercury options.
- Pennsylvania advises eating no more than one meal a week of sport fish caught in state waters, with stricter limits for some waters.
- Fish also has real health benefits, so the goal is choosing well rather than cutting it out.
How mercury gets into fish
Mercury released into the air settles into lakes, rivers and oceans, where bacteria convert some of it to methylmercury. Small fish take it in, bigger fish eat the small fish, and the levels build up the food chain. That is why large, long-lived predators tend to carry the most.
According to the Agency for Toxic Substances and Disease Registry, most people are exposed to methylmercury through food such as fish, seafood and rice.1 Very high exposures have caused tremors, poor coordination and vision problems, and birth defects in communities with heavy dietary exposure. Those were extreme situations. The everyday question for most people is how to keep exposure low while still eating fish.
Which fish carry the most mercury
The FDA and EPA sort commercial fish into "Best Choices", "Good Choices" and "Choices to Avoid". The highest-mercury group includes king mackerel, marlin, orange roughy, shark, swordfish, tilefish from the Gulf of Mexico, and bigeye tuna.2
| Group | What it means | Practical use |
|---|---|---|
| Best Choices | Lowest mercury | The default for regular meals, including in pregnancy |
| Good Choices | Moderate mercury | Eat less often than Best Choices |
| Choices to Avoid | Highest mercury | Skip in pregnancy and for young children, and keep rare for everyone else |
For adults outside pregnancy, the evidence on low-level mercury is less clear. A large review in JAMA concluded that the health effects of low-level methylmercury in adults are not clearly established, and that people eating fish five or more times a week should limit the species highest in mercury.3 The same review found that modest fish intake, about one to two servings a week, was linked to a lower risk of dying from heart disease.
Pregnancy, breastfeeding and children
A developing brain is the most sensitive to methylmercury, so the federal advice is aimed at people who are or might become pregnant, people who are breastfeeding, and children ages 1 to 11. For pregnancy and breastfeeding, the FDA advises 2 to 3 servings a week from the Best Choices list, with a serving of 4 ounces. Children need smaller servings matched to their age.
The advice is not to avoid fish. Fish provides omega-3 fats and other nutrients that support a baby's brain development. The aim is to choose lower-mercury options and avoid the highest ones.
Pennsylvania sport fish advisories
Commercial fish advice does not cover what you catch yourself. For that, Pennsylvania has its own system. The state's general advisory is to eat no more than one meal, about one-half pound, per week of sport fish caught in Pennsylvania waters.4 Stricter advice applies to certain species and waters, listed in the advisory tables published with the fishing summary.
The Pennsylvania Department of Environmental Protection notes that mercury and PCBs have been found in some fish from certain waters. Women of childbearing age, children and people who eat sport fish often are asked to be especially careful to space meals out.
Advisories also change and are not only about mercury. In October 2026 the state issued a "Do Not Eat" advisory for all fish species in the Wissahickon Creek Basin in Montgomery and Philadelphia counties, because of a chemical called PFOS.5 Before you eat a local catch, check the current list for that water.
If you fish outside Pennsylvania where no advisory exists, the federal advice is to eat only one serving of that fish and no other fish that week.
When to think about a mercury test
Most people who follow the fish advice do not need a mercury test. It is worth discussing with a clinician if you eat high-mercury species often, eat a lot of locally caught fish from waters with advisories, or have unexplained tremor or numbness along with a diet heavy in those fish. Pregnancy planning with a heavy fish habit is another reason to ask.
Mercury comes in different forms, and the right specimen depends on the form you are likely exposed to, as explained in blood vs urine vs hair metal testing. A result also needs to be read against your diet, since a recent tuna steak can change it.
Your next step
To understand what a mercury result can and cannot tell you, read mercury test results explained. The heavy metals testing hub covers other metals and how testing works.
Frequently asked questions
Is canned tuna safe to eat?
Canned light tuna is on the FDA's Best Choices list. Albacore (white) tuna is higher in mercury and sits in the Good Choices group, so it is better eaten less often.
Are stocked trout covered by the Pennsylvania advisory?
The statewide advisory applies to sport fish caught in Pennsylvania waters. Check the current Pennsylvania advisory tables for the species and water you fish, since some have stricter limits.
Do dental fillings matter as much as fish?
Amalgam fillings release a different form of mercury, elemental mercury. Fish is the main source for most people. If you have concerns about fillings, talk with your dentist rather than having them removed on your own.
Related reading
This article is for general education and is not medical advice. Decisions about testing and treatment should be made with your clinician. If you have severe symptoms, call 911. For a suspected poisoning, call Poison Control at 1-800-222-1222.
