During accidental releases, residents often stand in the street watching the plume of chemicals, said Michael Kint, hazardous materials ombudsman for the county. The success of sheltering in place depends on "people being prepared and knowing what to do beforehand.""We know there are people that don't pay attention to shelter in place," he said. "It is a constant struggle to continually educate people."
In a county of 1 million people, less than 20 minutes from San Francisco, the monthly turnover of residents can exceed 25,000, according to Randy Sawyer of the county's hazardous materials program. Contra Costa County also has a high population of Hispanic immigrants as well as 10,000 Laotians who speak a variety of dialects.
Each year authorities spend $500,000 to $600,000 on educating the public.
Cars not safe
"We're not 100 percent by any means, but we did a survey after the latest release, and 70 percent knew what to do," Sawyer said. "Now, why they know is kind of sad — it's because of all the accidents."
Most residents recognize that cars provide very little protection from toxic chemicals, Sawyer said. During the 1993 sulfuric acid release, many of the 20,000 injured were drivers who passed through the cloud on the highway.
Contra Costa County has had more than 60 accidents since 1989, ranging from hydrogen sulfide and chlorine releases to fires at plastic plants and oil refineries. Some instances have resulted in residents seeking medical help for symptoms ranging from respiratory problems to vomiting and headaches.
Not all events require sheltering in place but Kint says the county often will err on the side of caution.
Much of the community's awareness about chemical accidents and sheltering in place came after the 1983 Union Carbide accident in Bhopal, India that killed 3,800 people and injured thousands more.
The National Institute for Chemical Studies was founded in West Virginia after that incident to serve as a bridge between the chemical industry and the public.
The group preaches shelter in place to communities across the country with chemical plants.
The best protection
"It is the best way to protect the public in a chemical accident," said Mark Scott, director of NICS.
Although most emergency planners agree that houses provide more protection than cars do, older houses with drafty windows and doors cause concern among some Contra Costa County residents and officials.
Community activist Denny Larson, of Communities for a Better Environment, said he believes sheltering in place does work in newer homes, but it is used to convince poorer residents that they are safe living in the shadow of a major industrial plant.
Sawyer said some of those residents have asked for relocation.
In Deer Park, Texas, Jack Beckham, the city's director of emergency services, will talk individually to explain why staying in a home, even if it dates to World War II, is a better option than trying to leave.
"Once you explain it, it is common sense that you don't try to outrun a plume," he said.
Deer Park, just east of Houston, has 16,000 residents and 14 chemical plants producing harmful compounds such as chlorine and phosgene. Beckham said he has been educating residents about shelter in place for 10 years. To shelter in place goes against the natural instinct to run.
Officials in Contra Costa County say they will have to devise evacuation plans for some scenarios, such as a release of chlorine gas, which was used as a chemical weapon in World War I, or hydrogen fluoride, which goes through the skin and can pull calcium out of bones.
Sawyer said a limited evacuation might be necessary — after everyone has sheltered in place.
"The focus clearly has been on shelter in place," said County Supervisor John Gioia. "There's no detailed evacuation plan."
Shelter in place provides a simple, understandable message, he said. "In practice, the No.1 challenge is community education."
In June, the National Institute for Chemical Studies released a report entitled "Sheltering in Place as a Public Protective Action" that detailed several instances in which shelter in place was used effectively. Towns included in the report are in Arkansas, California, West Virginia, Texas, Nevada, Tennessee and Louisiana.
The report also detailed some of the Army's testing of in-place shelters.
The Army subjected 12 different kinds of homes, from trailers to brick houses dating from the 1920s, 1930s and 1950s, to toxic chemicals, in order to measure the usefulness of sheltering in place. In some cases, sheltering reduced exposure to toxic agents from 400 parts per million to 60 parts per million.
When the Army tested sarin on homes, it found that the construction materials absorbed some of the nerve agent. The tests demonstrated that after a toxic plume had passed, residents needed to ventilate their homes to clear out hazardous vapors.
Sheltering in place is not always the best course of action, Scott said. Evacuation, when possible, is preferable.
"There will be some infiltration after several hours, and the concentrations will build," 
said Sawyer of Contra Costa County. But he said that is preferable to a car, in which residents are almost certain to be exposed.
Deer Park's Beckham said public acceptance of shelter in place is only a matter of intensive community education.
"It's a matter of killing the doubting Thomases," he said. "With all the hurricanes down here, we have learned about mass evacuation, and I can tell you that it doesn't work."
Wade Mathews, of Tooele County Emergency Management in Utah, demonstrates the use of a shelter-in-place kit by taping around door edges and taping precut plastic sheeting over windows and vents.
