Mercury in fish is a real hazard - and a widely misunderstood one. Here is what the science actually says.
## Why mercury accumulates in fish
Atmospheric mercury (mostly from coal-burning) settles in ocean water, where bacteria convert it to methylmercury - the neurotoxic form. Methylmercury accumulates in fish tissue over their lifetimes. Larger, longer-lived predators accumulate the most.
## The FDA/EPA guidelines
Updated 2017 guidance categorises fish into three groups:
**Best Choices** (2-3 servings/week): salmon, sardines, anchovies, tilapia, catfish, pollock, cod, oysters, shrimp, scallops.
**Good Choices** (1 serving/week): tuna (canned light), snapper, halibut, sea bass, mahi-mahi.
**Avoid** (pregnant women and children): king mackerel, marlin, orange roughy, shark, swordfish, tilefish (Gulf of Mexico), bigeye tuna.
## Why the guidance targets specific groups
Methylmercury crosses the placenta and blood-brain barrier. It affects developing nervous systems more than adult ones. That is why guidance is stricter for pregnant women and children - not because fish is dangerous to healthy adults in normal servings.
## The omega-3 tradeoff
Skipping fish entirely to avoid mercury is worse than eating low-mercury fish regularly. Multiple studies show omega-3 intake benefits outweigh mercury risks at moderate consumption.
## Testing your own risk
Adults concerned about their mercury exposure can request a hair mercury test - non-invasive, cheap, and reflective of 3-6 months of dietary intake.