Prolonged exposure to crude oil and chemical dispersants is a public health danger, environmental scientists warned yesterday as BP spent a third day trying to initiate a "top kill" operation to cap the ruptured well on the sea bed.

The oil firm moved to a second stage of the procedure by injecting material such as golf balls, shredded tyres and rope into the well. But John Pack, a spokesman for BP, said it would not be clear until tomorrow if it would work. "We have never said there is a deadline or a schedule," he said. "We need to take this pretty slowly, but everything is going according to plan."

BP's beleaguered chief executive, Tony Hayward, yesterday drastically scaled upwards his assessment of the spill in the Gulf of Mexico. "This is clearly an environmental catastrophe. There is no two ways about it," he told CNN. "It's clear that we are dealing with a very significant environmental crisis and catastrophe." In an interview with the Guardian two weeks ago, he had described the oil spill as "tiny" relative to the size of the gulf.

"The amount of volume of oil and dispersant we are putting into it is tiny in relation to the total water volume," he said then.

However he was optimistic that the "top kill" stood a good chance of success, which he put at 60-70%. "We have wrestled it to the ground, but we haven't put a bullet in its head yet," Hayward said.

With no immediate end in sight, there were growing concerns over the effects on public health of a prolonged exposure to the oil as well as to the more than 3,640,000 litres (800,000 gallons) of chemical dispersants sprayed on the slick.

Environmentalists and fishing groups in Louisiana say prolonged exposure to the oil, in the form of tiny airborne particles as well as dispersants, could be wreaking devastating damage on public health.

They also accuse BP of threatening to sack workers who try to turn up for clean-up duty wearing protective respirators, and the Obama administration of refusing to release results of air and water quality tests that would show the impact of crude oil and dispersants on the environment.

Wilma Subra, a chemist who has served as a consultant to the Environmental Protection Agency, said there was growing anecdotal evidence that locals were falling ill after exposure to tiny airborne particles of crude. Air quality data released earlier by the EPA suggested the presence of chemicals that – while still within legal limits – could be dangerous. But Subra complained that the EPA was not releasing all data it had gathered from BP.

"Every time the wind blows from the south-east to the shore, people are being made sick," she said. "It causes severe headaches, nausea, respiratory problems, burning eyes and sore throats." Long-term health effects include neurological disorders and cancer.

Subra said there was even greater concern for those recruited to lay booms and skim crude off the water, since they were in closer proximity to the oil and the chemical dispersants.

Clint Guidry, of the Louisiana Shrimp Association, has accused BP of threatening to sack workers who turn up wearing respirators. The oil firm said it was not aware of any workers being turned away, but noted that it was the responsibility of the Obama administration to decide whether such protective gear was warranted.

Hugh Kaufman, chief investigator for the EPA's ombudsman, said he encountered similar worker safety policies after 9/11. "If people are wearing respirators, it scares people because they realise how toxic it is," he said "The administration is down-playing the problem because it saves them money down the line. It was the same at Ground Zero."

EPA tests indicate that the combined effect of dispersants and crude oil are even more toxic than individually. "There are dispersants being applied by aeroplane and by boat, and these people on the water are being sprayed over and over again," Subra said.

Five offshore rigs have been shut down since the spill after workers fell ill. Seven workers on a boat trying to scrub the oil from Breton Sound were taken to hospital on Wednesday, complaining of burning eyes, headaches, nausea, dizziness and chest pains. Five were treated and released.

Administration officials suggested in a conference call with reporters that the workers could be suffering from sunstroke in the hot Louisiana temperatures.

They also said the workers recruited for the clean-up – fishermen and shrimpers put out of work because of the spill – had received training and wore protective gear. However, the protective clothing does not include respirators, which environmental activists say violates safety regulations for workers exposed to dangerous chemicals.

None of the workers pictured raking up oil from the beaches of Grand Isle Louisiana in photographs released by the Deepwater command centre today was wearing a mask or respirator.