Mercury, fish and how much is truly how too much

As in many countries worldwide, South Africa enforces limits and restrictions on various food-related aspects. These include microbiological standards, font sizes on labels, permitted preservatives, and maximum levels of heavy metals in foods. Why? To keep us safe—within reason.

Mercury is one such heavy metal with permissible limits in certain foods. In South Africa, these limits are outlined in Regulation R358 of the Foodstuffs, Cosmetics and Disinfectants Act, 1972 (Act 54 of 1972): Regulations relating to maximum levels for metals in foodstuffs.

Why is this important? Because heavy metals (such as mercury) cannot be filtered out of the human body. Instead, they accumulate over a lifetime. Mercury occurs naturally in the Earth’s crust and is used in everyday items including batteries, thermometers, electrical switches, fluorescent lightbulbs, and is also present in fish.

There are three types of mercury harmful to humans: elemental, inorganic, and organic. This article focuses on organic mercury, which is found in the Earth’s crust. Exposure occurs through:

1. Fumes from coal mines

2. Fish that have ingested methylmercury (from water and soil)

3. Older antiseptics such as red mercurochrome, colloquially called monkeyblood in South Africa

Image depicting how mercury is accumulated in the food-chain. Image source: Priyadip Das

But what’s the real concern? Humans have eaten fish for millennia, so why worry now? The difference lies in lifespan and consumption patterns. Today, people live decades longer than 150 years ago, giving mercury more time to accumulate in the body.

Five side effects of accumulated organic mercury include:

• Numbness or dull pain in parts of the body

• Tremors (uncontrollable shaking)

• Double or blurred vision; blindness

• Seizures

• Developmental delays in infants

The limit for mercury in South Africa is 1mg/kg. In Europe, however, the limits are stricter. This raises questions: Are Europeans at greater risk? Are South Africans at lower risk? The answer lies in consumption.

South Africans consume approximately 6–8 kg of fish per person per year. In contrast, Europeans consume 22.9–23.5 kg per person per year down from 2022 consumption patterns. Given these figures, it makes sense that South Africa’s limits are higher—our average intake is less than half that of Europe. Therefore, our risk of fatal mercury accumulation is lower. That said, vulnerable groups should avoid fish species prone to high mercury levels.

Vulnerable groups include:

• Pregnant women

• Breastfeeding women

• Individuals with kidney disease

  
  

We are exposed to potentially harmful substances daily. Understanding how they affect us equips us to make safer, more informed choices. To find out more, click on https://youtu.be/7nvlZTb6voU?si=rlMTS-cIwYmB94BE or https://www.linkedin.com/feed/update/urn:li:activity:7437012869415833600

Until next time

Shelldon Athena Breda, registered dietitian, M.Sc.