CANCER-causing ratioactive chemicals may have been released into the air by the California wildfire, experts have warned.
The Woolsey Fire is thought to have began at the site of one the worst nuclear disasters in US history.
It has so far destroyed at least 370 homes and forced 170,000 people to evacuate around Los Angeles.
The inferno began on Thursday afternoon at the Santa Susana Field Laboratory (SSFL) — the site of a previous nuclear meltdown, decades of chemical spills and the illegal dumping of toxic waste.
Fears are mounting that harmful toxins are now being spread across the region as the blaze rages on.
The California Department of Toxic Substances Control (DTSC) had signed legally binding agreements to clean up the site by 2017 — but as of last month it had not begun the process.
Contamination experts and campaigners have accused the DTSC of a cover-up after it was was quick to release a statement denying people are at risk.
Around 10 hours after the fire began on Thursday, DTSC said its scientists "do not believe the fire has caused any releases of hazardous materials that would pose a risk to people exposed to the smoke."
But Melissa Bumstead, whose daughter Grace, eight, has twice survived leukemia that she believes was caused by contamination from the lab, described the fire as her “worst fear realised”.
She told The Sun Online: “The area is contaminated with radioactive waste.
Melissa has started a petition to try and get authorities to learn up the area, which has so far garnered over 450,000 signatures.
Doctors have also warned that the fire - which destroyed the homes of dozens of celebrities including Robin Thicke, Gerard Butler and Miley Cyrus - may have spewed dangerous chemicals into the air over Los Angeles and Ventura counties.
Dr Bob Dodge, President of Physicians for Social Responsibility Los Angeles, told The Sun Online: "The Woolsey Fire likely released and spread radiological and chemical contamination that was in SSFL’s soil and vegetation via smoke and ash.
"We know what substances are on the site and how hazardous they are.
"We’re talking about incredibly dangerous radionuclides and toxic chemicals such a trichloroethylene, perchlorate, dioxins and heavy metals.
"These toxic materials are in SSFL’s soil and vegetation, and when it burns and becomes airborne in smoke and ash, there is real possibility of heightened exposure for area residents."
The lab was set up to test rocket engines for the Mercury, Gemini, Apollo and Space Shuttle missions — but became infamous for a series of toxic waste scandals.
In 1959 a sodium reactor had a partial nuclear meltdown just 1,000 yards from where Woolsey Fire appears to have begun.
And in 1994 a chemical blast at the site killed two scientists.
Federal agencies found the company running the lab, Rocketdyne, had been dumping and burning hazardous waste under the guise of fake experiments.
Locals began to notice the area was becoming a hotspot for chronic illnesses — and fears grew that contaminants from the site may have affected the surrounding towns.
Research found that people living within a two-mile radius of the site were 60 per cent more likely to be diagnosed with certain cancers than those living five miles away.
SSFL was shut down in 2006 and the 2,850 acre facility was bought by aerospace giant Boeing.
In 2010 a local campaign group, the Rocketdyne Cleanup Coalition, succeeded in forcing DTSC to sign an agreement with NASA and the Department of Energy to clean up the site by 2017.
But campaigners said officials have yet to take any action.
Dan Hirsch, former director of the Program on Environmental and Nuclear Policy at the University of California, Santa Cruz, said that if pollutants on the site were ingested, they could lead to cancer, neurological diseases, miscarriages and birth defects.
He said: "This is one of the most contaminated sites in the country.
"It is the location of a partial nuclear meltdown, several other reactor accidents, numerous spills, and tens of thousands of rocket tests.
"The property is heavily contaminated with a witches’ brew of radioactive materials and of toxic chemicals."
The fire was first reported at 2.24pm on Thursday on E Street and Alfa Road on the Santa Susana site, just north of Bell Canyon in Canoga Park.
Electricity firm Southern California Edison reported that just two minutes earlier an electrical substation on the lab site tripped.
The report, sent on Thursday evening to the California Public Utility Commission, was the first piece of evidence that Chatsworth electric substation at the lab may have been the source of the blaze.
DTSC said it will check air monitoring machines once the fire department allows access to the area.
A statement said: "The fire agencies responding to the fire have consulted with their own hazardous materials coordinator who is familiar with the site and determined the fire did not present any risks other than those normally present in a wildfire situation.
"DTSC will continue to update the communities around the SSFL site as new information becomes available about the site and the impacts of the fire.”