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– John D. Brider was found passed out near a homeless shelter and taken to Los Angeles County-USC Medical Center, where he later died.

Brider, 63, had gone into cardiac arrest and oxygen had been cut off to his brain. But another, seemingly improbable, factor contributed to his death last winter: hypothermia, or loss of body heat, from being out in the cold, the Los Angeles County Department of Medical Examiner ruled.

One of the abiding myths about Los Angeles is that homeless people come here from the East Coast or Midwest because at least they won’t freeze to death.

But despite L.A.’s typical sunshine and mild weather, five homeless people, including Brider, died of causes that included or were complicated by hypothermia in the county last year, surpassing San Francisco and New York City, which each reported two deaths.

The Los Angeles County coroner said 13 people died, in part, of the cold over the last three years. Advocates worry that this cold, rainy winter will bring more deaths.

Hypothermia led to more deaths in L.A. than in colder regions because 39,000 homeless people here live outdoors — by far the most of any metropolitan area in the country. Hypothermia can set in at temperatures as high as 50 degrees, experts say.

Going without a hat can drain up to half a person’s body heat, and wet clothing can intensify heat loss twentyfold, according to a 2007 report from the National Health Care for the Homeless Council. Underlying medical conditions, alcohol and drug use — including the use of psychiatric medications — mental illness and the privations of living outdoors intensify the risk. Brider, for example, tested positive for cocaine and had cancer of the throat and tongue, the coroner said.

“Many people experiencing homelessness suffer from malnutrition and sleep deprivation, leading to some of them remaining out in the cold. Ultimately, sometimes they die,” said Bobby Watts, the homeless council’s chief executive.

L.A.’s hypothermia cases are a tiny fraction of the overall homeless death toll, which climbed from 720 in 2016 to 900 last year. But hypothermia is a particularly preventable way to die.

A spokesman for Mayor Eric Garcetti said the city and county added 1,607 new shelter beds in a year and expanded outreach. The county’s winter shelter program provides 1,200 extra beds from December to the end of March.

“The number of emergency beds for our homeless neighbors has increased each year for the last three years,” said spokesman Alex Comisar.

But although most cold-exposure deaths occur in the winter, Mark Stuart, 56, died on a Long Beach embankment of probable hypothermia in April 2016 — after the winter shelters shut down.

Jim O’Connell, founding director of the Boston Health Care for the Homeless Program, said hypothermia is a particular risk when the temperature drops more than 10 degrees in one day, a common phenomenon in L.A. Jonathan Sherin, director of the Los Angeles County Department of Mental Health, said the homeless with severe and persistent mental illness are in most jeopardy.

Over the last six to eight months, the county’s specialized outreach teams, with 30 staff members supported by a psychiatrist, have fanned out to remote encampments to find homeless people who need help. But with the persistent rain this winter, hypothermia is the immediate concern. Winter shelters, which usually close by day, stay open around the clock during heavy multiday storms.

“The idea that people froze to death is really horrible; It is a shared societal tragedy,” O’Connell said.