The Bay Area air district declared toxic chemical levels were safe after Monday night's Chevron refinery fire in Richmond, so why did nearly 1,700 people rush to hospital emergency rooms complaining of stinging eyes, wheezing and difficulty breathing?
Experts note the tests analyzed chemicals in the air, but not the particulate matter -- soot -- that turns smoke black and can irritate the eyes, nose, throat and lungs.
"The fact that the laboratory results don't show much is reassuring, but that's not the whole story," said Dr. Wendel Brunner, public health director for Contra Costa County. "The tests that we were doing didn't really test for the particulate matter."
The plume of smoke rose 1,000 feet or higher and blew east into the Central Valley and the Sierra, dispersing along the way, said Eric Stevenson, director of technical services for the Bay Area Air Quality Management District.
As a result, most of the soot lifted above the community, he said.
But Brunner noted that winds "were quirky that night" and may have blown particulate matter through some neighborhoods before rising.
The air district tested for sulfur dioxide and hydrogen sulfide and found ordinary levels except for a small increase in sulfur dioxide in Martinez, but still well within safety standards, Stevenson said.
The air district also tested for 23 hydrocarbons as the fire burned and "there was nothing out of the ordinary that occurred," he said.
Many residents, however, say their experience was anything but ordinary.
"It felt like an asthma attack, and I haven't had one of those in years," said Jamisha Reames, 23, of Richmond. "I had to give my son a breathing treatment, and my newborn daughter was coughing a lot."
"I had tightness in my chest and a bad cough," said Charmagne Moore, 36, of San Pablo. "I didn't want to leave the house."
Moore visited the emergency department at Kaiser Permanente in Richmond on Tuesday and was prescribed two inhaled medicines, something she had never used before.
By midday on Wednesday, Doctors Medical Center in San Pablo estimated it had seen more than 600 people complaining of respiratory problems. Kaiser Permanente Richmond Medical Center said it had seen more than 900 people.
Some may have sought care to get documentation for filing a claim against Chevron, but health leaders said they believe many of the complaints were real.
"A lot of people were understandably anxious," Brunner said. "Fortunately, nobody was injured seriously enough to have to be admitted to the hospital."
The air district had instruments that measured particulate matter several miles away in Oakland, Vallejo and San Rafael and that turned up nothing out of the ordinary, Stevenson said.
Other equipment in San Pablo, a couple miles from the refinery, can also measure particulate matter, but the results from that will not be available for a week or two, he said.
Brunner noted that the San Pablo equipment is on an automatic schedule and would have begun monitoring at midnight, several hours after the fire began.
"So we are not going to have really good measures of the particulate matter," Brunner said. "The truth of the matter is we are not going to have good measures of what people were exposed to."
Although the level of toxic chemicals in the air remained well within safety standards, people can experience a cumulative effect from living in an area with industrial and diesel emissions, experts note.
Young children, the elderly and those who have chronic conditions may be more affected by low levels of exposure, said Stephen Zavestoski, an El Cerrito resident and professor of environmental studies at the University of San Francisco. "The cumulative effect of this really triggers people's emotions and their anger," he said.
People living in industrial areas such as North Richmond and West Oakland often contend with higher rates of diesel and other emissions, asthma hospitalizations, poverty and a lack of educational opportunities that can bring added stress and increase health problems, noted Alex Briscoe, director of the Alameda County Health Care Services Agency.
"It's not the exposure of this week," Briscoe said. 'It's the exposure over months and years and decades."
Staff writer Sean Maher contributed to this report. Sandy Kleffman covers health. Contact her at 925-943-8249. Follow her at Twitter.com/skleffman.