Although hydrogen sulfide frequently smells like rotten eggs, the gas can knock out the body's smell receptors, "and the smell goes away, so people don't realize they're still in a poisoned atmosphere," Casavant said. "Victims have plenty of oxygen in their blood, but because their cells can't use the oxygen, they gasp, choke, and struggle for air."
Hydrogen sulfide gas is among agents used in so-called chemical suicides, in which people deliberately mix particular household chemicals in an enclosed space like a car to end their lives. Law enforcement officials have seen a growing number of these cases in this country, which followed a spate of cases in Japan in 2008.
Both Johnson-Arbor and Casavant said that another possibility was carbon monoxide, a colorless, odorless gas that could have backed up from a blocked chimney flue, a leak in a ventilation pipe "or someone using gasoline-, propane- or kerosene-powered equipment or generators indoors," Johnson-Arbor said.
Casavant also floated the possibility of a deliberate act. "In this day and age, first responders should also think about the possibility of terrorism and the intentional release of a poison into that atmosphere," he said. "The sarin used in the Tokyo subways in March 1995 caused some people to lose consciousness, and left many people struggling to breathe."
But Simmons said the terrorist scenario was unlikely because people only became sick if they breathed the air in the women's restroom.
Pooler firefighters who arrived at the McDonald's soon learned that people who had tried to help Felton and Barry began falling ill themselves. One of the firefighters treating the women soon began feeling woozy, "like the walls were starting to move in and out on him. He started noticing respiratory problems," Simmons said.
Firefighters then donned special air filtering masks to protect themselves. Three of them were among the 10 people taken to a Savannah hospital for evaluation, although after being checked, all three were back at work Wednesday evening.
But one of the firefighters continued to have some residual effects from the episode. He reported still having a sore throat and irritation in his chest and visited his doctor today as a precaution, Simmons said.
Robert Vazzi, area director of the Savannah, Ga., office of the federal Occupational Safety and Health Administration, said Friday that his office had begun an investigation, which he expected could last a while. OSHA was called into the case to investigate the restaurant workers' potential exposure to the chemicals and to investigate "any safety violations that might have occurred that might have contributed to the accident."
"Hopefully, we'll be coming up with something," Vazzi said. "We may not be able to find anything."