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.
Source: BBC
