As we become increasingly aware of toxins lurking in our food—including protein powder, dark chocolate, and brown rice—it’s only natural to wonder about other foods you regularly eat. One of those foods might be fish, some of which is known to be high in mercury—a neurotoxin. And while chowing down on tuna salad or a sushi roll you may be wondering: How much mercury is too much?

“All you have to worry about is, don’t eat high mercury fish several times a week,” food scientist Abbey Thiel tells Fortune.

Mercury, according to the Environmental Protection Agency, is a both a naturally occurring element (from volcanic activity) and a byproduct of industrial pollution, coal burning, power plants, and burning fossil fuels, which disperse the toxin into the air for it to be deposited back into water bodies and ground sediments that convert it into an even more toxic form, known as methylmercury.

High levels of mercury exposure can harm the brain, heart, kidneys, lungs, and immune system of people of all ages—but it is considered the greatest threat to infants in the womb, whose brain and nervous system development is at risk, and children, who could face worsened cognitive thinking, memory, attention, language, fine motor skills, and visual spatial skills.

Fish are the primary food source of mercury, Thiel says, and thankfully, there are only a handful of culprits to keep track of.

Which fish are highest in mercury

Fish that are high up on the food chain are the ones that will have the highest mercury levels, explains Environmental Working Group senior scientist Tasha Stoiber.

Those include:

• King mackerel

• Shark

• Swordfish

• Marlin

• Tuna

They have the highest levels because mercury bioaccumulates in their bodies from eating smaller fish that already have mercury in their systems as well.

“Sometimes they eat hundreds of these other fish in one gulp,” Thiel says. When mercury enters a fish’s body, Thiel explains, it binds to their muscles and proteins and remains in their body, continuing to build up as that fish consumes other fish with mercury. “When we eat those proteins, then we now have it in our body, and it can bind to our different proteins and muscles,” she adds.

How much high-mercury fish is safe to eat

For the predatory fish known to be highest in mercury, Stoiber recommends eating them “very infrequently or not at all.”

The Centers for Disease Control and Prevention follows the Food and Drug Administration’s chart of fish highest and lowest in mercury, providing the following guidance for pregnant and breastfeeding women and young children:

• Eat a variety of fish.

• If you eat fish caught by family or friends, check for fish advisories. If there is no advisory, eat only one serving and no other fish that week.

• Try to avoid eating the “Choices to Avoid” fish (found on the FDA chart) or feeding them to children. It is best to eat a variety of fish from the “Best Choices” and “Good Choices” categories on the chart.

And for adults:

• Eat 2 to 3 servings (between 8 and 12 ounces; 1 serving = 4 ounces of fish measured before cooking) of fish a week from the “Best Choices” list OR 1 serving (4 ounces) from the “Good Choices” list on the FDA chart.

“Mercury messes up how the brain will develop,” Thiel says. “Young kids should be very careful, and pregnant mothers and nursing mothers.”

Swap for fish that are lowest in mercury

Smaller, non-predatory fish are your best bet to reap the nutritional benefits of seafood and avoid too much mercury, experts say.

“These little fish, they just don’t eat the amount that a large predator fish does,” Thiel says. “Yes they do have mercury, but it’s such a considerably lower amount than other fish.”

You can check the FDA’s chart for a full list, and the EWG also has a seafood guide of the safest choices. Low-mercury fish include:

• Cod

• Salmon

• Sardines

• Tilapia

• Haddock

• Trout

When eating those fish, “You could have 2-3 servings of fish [per week] and not even worry,” Thiel says.

The omega-3s in fish like salmon could be doing heavy lifting to combat the negative effects of mercury, Stoiber adds. “If you’re eating fish higher in omega-3s, the omega-3s act protectively—that is actually really good for brain development,” she says.

Stoiber learned this from a study the EWG conducted in 2018, which found that switching to lower-mercury fish not only drastically reduced the mercury levels in the body in just three months, but also acted as a protective mechanism for children’s brain development because of omega-3s’ known benefits.

“Eating lower mercury is ideal,” Stoiber says. “That combination of lower mercury plus omega-3s is where you get the most benefit.”