Before you have your next meal, you may need to ask where the vegetables you have with it are from.

Chances are you got them from your nearest supermarket or from your favourite ‘Mama Mboga’, but do you know you could be exposing yourself to heavy metals that are harmful to your health?

An investigation by the Star has found that you are better off buying sukuma wiki (kales) from your local market than at a supermarket, following intensive testing of samples from various food markets in the city, as well as two major supermarkets.

The one kilogramme kale samples were collected from Gikomba, Marikiti, Korogocho market in Kariobangi, Kangemi and Githurai markets, as well as from Naivas supermarket in Westlands, and Nakumatt supermarket in Lavington.

They were subjected to lab analyses at the Kenya Plant Health Inspectorate Service (Kephis) and at a private laboratory, Analab Limited.

The tests were done in two different labs for verification purposes and also to determine a trend in the level of metals in sukuma wiki in Nairobi markets.

The tests conducted were to determine the presence (and levels) of lead, mercury, iron, copper and cadmium in kales from Nairobi main markets and supermarkets.

The lab analyses done at Kephis indicate that the kales bought from the two local supermarkets contained 0.13mg per kilo and 0.15mg per kilo of mercury respectively.

This is higher than the recommended World Health Organisation limits of 0.1mg per kilo. The samples from the markets had 0.007mg per kilo and a maximum of 0.1 mg per kilo, which were within WHO acceptable limits.

The level of mercury found in kales from Kariobangi market, however, was also higher at 0.11mg per kilo.

Samples from Kangemi, Gikomba, Githurai and Marikiti markets were within limits at 0.06, 0.07, 0.05 and 0.05mg per kilo respectively.

Tests carried out at Analab showed that the samples had mercury levels of 0.01mg per kilo, which is within the stipulated WHO limits. However, there were significant levels of lead in the kales from most markets in Nairobi.

Kales from Kangemi market in Westlands had a level of 0.20mg per kilo of kales, while Korogocho market in Kariobangi had 0.17mg per kilo. Githurai market had 0.08mg per kilo, and kales from Gikomba market had the highest levels of lead at 0.23mg per kilo.

However, Wakulima/Marikiti markets in the Central Business District showed significantly lower levels of lead in comparison to the rest, at 0.01mg per kilo.

While total mercury levels in animals and plants are usually very low, repeated exposure through consumption of contaminated foods can be dangerous, as methylmercury is toxic to the nervous system, kidney, liver and reproductive organs.

According to WHO’s fact sheet posted on its website in March 2017, mercury is a major threat to public health because even exposure to smaller amounts poses a threat to human development, especially of children in early stages of life.

Mercury may have toxic effects on the nervous, digestive and immune systems, lungs, kidneys, skin and eyes, severely impacting these functions and compromising the health of anyone exposed to significant amounts of it over time.

In August 2017, the Minamata Convention on Mercury came into force, committing its 74 Parties to reducing the risks to human health and the environment from the harmful release of mercury and mercury compounds.

Kenya became a party to the convention on October 10, 2013, and is now legally bound to ban new mercury mines, regulate the use of mercury in artisanal and small-scale gold mining, manufacturing processes, and the production of everyday items such as cosmetics, light bulbs, batteries and teeth fillings.