Swiss firms have been criticised in a report for their links to the African trade in diesel with toxin levels that are illegal in Europe.
Campaign group, Public Eye, says retailers are exploiting weak regulatory standards.
Vitol, Trafigura, Addax & Oryx and Lynx Energy have been named because they are shareholders of the fuel retailers.
Trafigura and Vitol say the report is misconceived and retailers work within legal limits enforced in the countries.
Three of the distribution companies mentioned in the report have responded by saying that they meet the regulatory requirements of the market and have no vested interest in keeping sulphur levels higher than they need to be.
Although this is within the limits set by national governments, the sulphur contained in the fumes from the diesel fuel could increase respiratory illnesses like asthma and bronchitis in affected countries, health experts say.
The picture is changing but there are still several African countries which allow diesel to have a sulphur content of more than 2,000 parts per million (ppm), with some allowing more than 5,000ppm, whereas the European standard is less than 10ppm.
Rob de Jong, from the UN Environment Programme (Unep), told the BBC that there was a lack of awareness among some policy makers about the significance of the sulphur content.
Swiss-based policy lobby group Public Eye said: “Swiss traders and others maximise profits by taking advantage of weak regulations to produce and sell harmful fuel. This form of regulatory arbitrage ignores the serious risks to public health.”
The report called on Swiss firms Trafigura and Addax & Oryx as well as Vitol, the world’s largest trading firm, to only sell fuel that meets higher regulatory standards.
The group described the issue as a “ticking time bomb” as conurbations develop across Africa and populations boom in cities such as Nigeria’s Lagos and Ghana’s Accra.
Vitol said it complied with all government regulations, and could not control by itself the quality of fuel sold at the pump. The others named did not immediately respond to requests for comment.
Regional countries alert
Kenya, Tanzania, Uganda and Morocco have increased fuel quality requirements.
But the African Refining Association, a non-profit group that represents the continent’s downstream sector, said that changing the actions of the trading houses alone would not fix the problem.
“If Swiss traders followed the report recommendation today their role would be filled by (possibly less reputable) traders from other nations,” the ARA said.
“The role of improving fuel quality in Africa clearly rests with African governments, not with the fuel suppliers.”
The ARA said it had been pressing governments in tandem with the United Nations Environment Programme.
But higher quality means higher costs, and with many countries facing severe shortages in public finances, they are wary of angering their populations with higher pump prices.
“The bottom line is that governments have competing priorities for available funds,” said David Bleasdale, executive director of CITAC Africa Ltd, a consultancy that focuses on the African downstream sector.