The company that discharged 500 tonnes of waste in Ivory Coast has denied that the product was toxic.
The Netherlands-based company, Trafigura, said independent analysis showed the chemical sludge met international safety standards.
Seven people died and 40,000 were treated in hospital for nausea, breathing problems and nosebleeds.
Ten people, including two French Trafigura executives, have been charged in connection with the discharge.
The environmental group Greenpeace has blocked the tanker concerned, the Probo Koala, in an Estonian port to demand a European inquiry.
Trafigura first attempted to discharge the chemical slops, which contains mercaptan, from one of its tankers, the Probo Koala in the Dutch port of Amsterdam in early August.
But the company that was to dispose of the waste suddenly increased its charges dramatically - asking for 40 times more to treat the waste.
Trafigura refused, and the tanker proceeded to Nigeria. There it tried to offload the waste, but again failed to reach an agreement with two local firms.
It was only in Ivory Coast that it managed to find a company to handle the waste at a cost the company would accept.
On 19 August the waste was discharged near the commercial capital, Abidjan. Two weeks later the first complaints arose.
Instead of being incinerated as it should have been, the waste had been dumped.
One of the chemicals it contained gave off a pungent smell, although the company insists it was not intrinsically unsafe.
But a director of Trafigura, Eric de Turkheim, in the first interview since the story broke, said he could not rule out a link between the waste and the health problems suffered by the people of Abidjan.
"We do not know whether there was any attempt to use the product in any way or form, which could have generated some toxicity."
Trafigura suspects that someone in Abidjan may have attempted to refine the chemical slops to try to recover the gasoline content, and this - in some way - turned it into hazardous waste.
In the meantime the company sent senior officials to Ivory Coast to offer help in the clean-up, and has provided medical assistance.
But two directors have been arrested by the Ivorian authorities, and are now facing up to 20 years in prison for their alleged part in this toxic waste scandal
- 116 Human Rights