The sharptooth catfish (Clarias gariepinus) – a source of food for people living along the Klip River in Gauteng – contains high levels of toxins that can cause cancer if eaten regularly.The river flows through Soweto, Fleurhof and Lenasia, where the catfish are consumed by residents daily. According to a recently published study in Science of the Total Environment, the fish contain high levels of pesticides that can be extremely harmful to humans. In some cases, the risk of contracting cancer is 1 000 times more than what is considered safe. News24 spoke to one of the study's authors, Professor Rialet Pieters, who is a researcher at the Unit for Environmental Sciences and Management at North-West University.According to Pieters, there is no way of controlling the pesticides found in the fish as they are residues that occur freely in the environment. In fact, with the exception of dichloro-diphenyl-trichloro-ethane (DDT), they haven't been in use since the 1980s."South Africa is a signatory to the Stockholm Convention [on Persistent Organic Pollutants] which means we do not use these pesticides anymore, except for DDT in areas that use it to control malaria."'Toxins can be found anywhere'"We have evidence that the presence of these pesticides is due to historic use. Most of these were banned in the 1980s. And the DDT that is still used in the northern parts of the country can be transferred by air – we call it long-range transport. "That means it can be transported to any part of the world. We have found these compounds in areas such as the Antarctic, in whales, for instance. People would never have used it in those areas."Pieters said once the high levels of toxins in the fish had been established, the next step was to do a human health risk assessment.Read: It's in your food - can this herbicide commonly used in SA give you cancer?It calculates an exposed individual's chances to develop cancer.If the risk is greater than one in 10 000, it is considered an "unacceptable risk", according to the US Environment Protection Agency."If someone living in Lenasia were to regularly eat contaminated fish, that person's risk jumps to 1 105 out of 10 000."Pieters said samples taken from the Lenasia area posed the highest risk. A mid-range estimate puts the risk of developing cancer at 251 in 10 000. "We got different values for Fleurhof and Orlando, ranging between 172 and 359 in 10 000 at Fleurhof and between 191 and 624 in 10 000 people at Orlando."Pieters says the contamination can cause "a wide number of cancers", among them non-Hodgkin's lymphoma as well as skin and lung cancer. "There are many more," Pieters said.'Actively fished'"The sharptooth catfish that we collected is actively being fished by people."According to Pieters, one of the study authors, Wihan Pheiffer, observed subsistence fishermen active along the Klip River. "He wanted to buy fish from them but they refused because it is a source of food for them."We are, however, not sure to what extent the fish is being consumed."That would be the next step: finding out how many people are eating this fish and how much of it they are consuming."Also read: Local shop allegedly sells rotten food products to the public'People don’t die immediately'According to Pieters, the risk of developing cancer is a long-term risk. "People don't die immediately [after consuming the fish]. The fish appear to be healthy and people are not concerned, as they would be if the fish were floating dead in the water. So people are unaware of these compounds in our water that can have all kinds of harmful effects."Nothing might happen now, but you can die of cancer in 15 to 20 years."Pieters said there was nothing anybody could do to control or avoid these contaminants from spreading. "Once they hit the atmosphere, we cannot control their spread at all."Pieters said public awareness and communication about the risks were the only way to protect people from exposure. "There are so many harmful compounds that people are exposed to daily: when you fill up your car and inhale the fumes; the smell of a new vehicle, which are vapours; the smell of a recently painted house; pharmaceuticals and cosmetics…those all contain compounds that can have different effects on your body."Pieters said although the body can metabolise many of these, most people are exposed to harmful compounds their entire lives. "Most people don't realise it. Just because you don't see an immediate effect, it doesn't mean constant exposure to a mixture of compounds won't harm you in 15 or 20 years," Pieters said.