Carl De Keyzer/Magnum Photos
Swiss firms have been criticized 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.
Why are regulations so lax?
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 Program told the BBC that there was a lack of awareness among some policy makers about the significance of the sulphur content.
For a long time countries relied on colonial-era standards, which have only been revised in recent years.
Another issue is that in the countries where there are refineries, these are unable, for technical reasons, to reduce the sulphur levels to the standard acceptable in Europe. This means that the regulatory standard is kept at the level that the refineries can operate at.
Some governments are also worried that cleaner diesel would be more expensive, therefore pushing up the price of transport.
But De Jong argued that the difference was minimal and oil price fluctuations were much more significant in determining the diesel price.
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