Earlier this week, concerns were raised about the safety of canned tuna after tests on 150 cans purchased in five countries found half contained more than the recommended level of mercury.
The study by French researchers found that all cans studied – including some purchased in Britain – contained some of the metal, which in large quantities can damage brain health and has been linked to cancer.
In fact, the NHS advises pregnant women not to eat more than two tuna steaks or four cans a week due to the risk of damage to an unborn baby’s brain.
And it’s not just tuna that contains mercury; in fact all fish, and some species, such as swordfish, are packed with particularly large amounts.
So how much can we safely eat?
First, it makes little difference whether the fish is canned or not.
Most of the mercury released into the atmosphere from natural and human causes, such as burning coal, ends up in the ocean, where some is converted by small organisms into a toxic compound known as methylmercury.
This methylmercury works its way up the food chain and accumulates in high concentrations in top predators.
But for just 65 cents per can, tuna can provide an affordable source of protein, healthy fats and vitamin D.
Because tuna – and other predators or longer-lived species such as sharks or swordfish – are higher on the food chain, they eat smaller fish and accumulate more mercury over time.
Exposure to methylmercury can damage the kidneys and nervous system, cause vision problems and increase the risk of cardiovascular disease.
According to the World Health Organization, mercury can also cause neurological and behavioral disorders if it is inhaled, eaten or comes into contact with the skin.
Symptoms include tremors, insomnia, memory loss, headaches, and cognitive and motor impairment.
Research has shown that some forms of mercury at very high doses have caused the development of several types of tumors in rats and mice.
But the International Agency for Research on Cancer (IARC) found there wasn’t enough evidence to determine whether mercury could cause cancer in humans.
The NHS advises people to eat at least two portions of fish a week to reap its nutritional benefits, including protection against heart disease and bone-boosting vitamin D.
Research has shown that the average person is unlikely to ingest enough mercury from fish to cause serious harm.
Lab tests conducted in 2015 for ABC TV’s science program Catalyst found that you would have to eat at least 25 cans of tuna a week before reaching the amount that research has shown is harmful.
That’s the equivalent of about 16 140g tuna steaks.
This is what scientists from the Australian government agency The Commonwealth Scientific and Industrial Research Organization (CSIRO) who conducted the research wrote in the Conversation that the exact amount depends on the brand of tuna and the weight of the person.
For example, Skipjack varieties are said to contain less mercury than albacore tuna.
They stressed that even the worst brand they tested “wasn’t that bad” and said consumers were unlikely to reach maximum safety limits.
Under current EU and UK legislation, the limit for mercury in tuna is 1 mg/kg and 0.3 mg/kg for other fish such as cod.
By comparison, canned salmon has much lower mercury levels, with an average of 0.05 micrograms of mercury per gram, according to the Global Salmon Initiative.
Rob Hobson, registered nutritionist and author of Unprocessed: Your Family Life, told MailOnline Eating ‘light tuna’ two to three times a week is safe and also encourages people to consume a variety of fish.
“Research shows that light tuna and skipjack tuna contain less mercury than larger species such as bigeye and albacore tuna,” he said.
‘The advice in the US from the FDA is that you should limit tuna intake to two to three portions per week and choose light tuna (skipjack) varieties.
Meanwhile, the NHS says some white fish, such as cod, can be eaten as much as you like due to their vanishingly low mercury content.
However, other white fish contain mercury as well as other pollutants and should be limited to a few times a week. Think of sea bream, sea bass, turbot, halibut and rock salmon.
According to the World Health Organization (WHO), pregnant women and children are particularly vulnerable to high levels of methylmercury.
You should not eat shark, swordfish or marlin if you are pregnant or trying to become pregnant, the NHS adds. But all other adults, including breastfeeding women, should eat no more than one serving per week.
This is because these fish can contain more mercury than other types of fish and can damage a developing baby’s nervous system.
The FSA told MailOnline: ‘Mercury is found in all fish due to natural accumulation from the environment, so it cannot be completely eliminated.
‘Instead, we aim to ensure levels are as low as reasonably achievable. Large, longer-lived predatory fish (e.g. a shark) have the highest mercury content.
‘In Britain we have a maximum mercury level in tuna of 1.0 mg/kg.
‘Importers and food business operators have a responsibility to ensure that the food they trade is safe for human consumption and will carry out their own testing to ensure compliance.’