Earlier this week, concerns were raised about the safety of tinned tuna, after tests on 150 cans purchased in five countries found half contained in excess of the recommended amount of the toxin mercury.

The study, by French researchers, found that all tins studied — including some bought in the UK — contained some of the metal, which, in large quantities, can harm brain health and has been linked to cancer.

In fact, the NHS advises pregnant women should not eat more than two tuna steaks or four tins a week due to the risk of harm to an unborn baby's brain. 

The FDA urges pregnant Americans to limit their intake to a maximum of three cans per week. 

And it's not just tuna that contains mercury; in fact all fish does, with some types, like swordfish, packed with particularly large amounts.

So how much is safe for us to eat?

Firstly, whether the fish is tinned or not makes little difference. 

The majority of the mercury put into the atmosphere from natural and human causes, such as burning coal, ends up in the ocean where some is converted by tiny organisms to a toxic compound known as methylmercury.

This methylmercury works its way up the food chain and accumulates in top predators in high concentrations.

As tuna — and other predators or longer-living species like sharks or swordfish — are higher up the food chain, they eat smaller fish and accumulate more mercury over time.

Exposure to methylmercury may damage the kidneys and nervous system, trigger issues with vision and raise the risk of cardiovascular disease.

According to the World Health Organisation, mercury can also cause neurological and behavioural disorders if it is inhaled, eaten or if the skin comes into contact with it. 

Symptoms include tremors, insomnia, memory loss, headaches and cognitive and motor dysfunction.

In very high doses, studies have found that some forms of mercury have triggered the development in several types of tumors in rats and mice.

But the International Agency for Research on Cancer ( IARC) found that there was not enough evidence to determine whether mercury could cause cancer in humans.

The NHS and CDC advise people to eat at least two portions of fish a week in order to reap its nutritional benefits like protecting against heart disease and bone-boosting vitamin D.

Studies have shown it is unlikely that the average person will consume enough mercury via fish to cause them serious harm. 

Lab tests conducted for the ABC TV science program Catalyst in 2015 revealed you'd have to eat at least 25 tins of tuna a week before you hit the amount that studies have shown is harmful. 

That's the equivalent of around 16 140g tuna steaks. 

The scientists at the Australian Government agency The Commonwealth Scientific and Industrial Research Organisation (CSIRO) who conducted the study wrote  in the Conversation that the exact amount depends on the brand of tuna and the person's weight. 

For instance, Skipjack varieties are said to be lower in mercury than albacore. 

They stressed that even the worst brand they tested 'wasn't that bad' and said it was unlikely consumers would reach upper safety limits. 

Under current EU and UK law, the limit for mercury in tuna is 1 mg/kg and 0.3 mg/kg for other fish such as cod.

In comparison, tinned salmon has much lower mercury levels with an average of 0.05 micrograms of mercury per gram, according to the Global Salmon Initiative. 

The FDA has a slightly different measurement, limiting the amount of mercury in fish to 1.0 parts per million (ppm). This limit is set to be ten times lower than the levels associated with adverse health effects, the agency says. 

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 urges people to consume a variety of fish. 

'Research suggests light and skipjack tuna are lower in mercury than larger species such as bigeye and albacore,' he said.

'The advice in the US by the FDA is that you should limit your intake of tuna to two to three servings per week and choose light tuna (skipjack) varieties.'

Meanwhile, the NHS states that some white fish such as, cod, can be eaten as much as you like due to vanishingly low mercury levels.

However other white fish contains mercury as well as other pollutants, and should be limited to a few times a week. This includes sea bream, sea bass, turbot, halibut and rock salmon.

Pregnant women and children are particularly vulnerable to high levels of methylmercury, according to the World Health Organization (WHO). 

You should not eat shark, swordfish or marlin if you are pregnant or trying to get pregnant, the NHS adds. But all other adults, including breastfeeding women, should eat no more than one portion 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 as a result of natural accumulation from the environment so it cannot be eliminated entirely. 

'Instead, we aim for levels to be as low as reasonably achievable. Large longer-lived predatory fish (shark, for example) have the highest mercury levels.

'In Great Britain we have a maximum level for mercury in tuna of 1.0 mg/kg.

'Importers and food business operators have a responsibility to ensure the food that they trade is safe for human consumption and will undertake testing themselves to ensure compliance.'