What killed the family of three and their dog had puzzled investigators since August.

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A family that mysteriously died in August on a California hiking trail likely died from a combination of hyperthermia and dehydration, law enforcement agents revealed on October 21.

Jonathan Gerrish, Ellen Chung, their 1-year-old daughter, Miju, and the family's dog. Oski, were all found dead on a steep hiking trail on August 17 about 1.5 miles away from where they had parked their car. The family had hiked in the Sierra National Forest near the Merced River, and they were reported missing by friends.

Investigators were unsure for weeks about what led to the deaths of the entire family, with one theory suggesting that exposure to water contaminated with toxic algae blooms led to their deaths. Water near the trail tested positive for toxic algae and included Anatoxin-a, a toxin that the Environmental Protection Agency says can kill people.

Cause of Death Released for Family Found Dead in Desert , A mother is hiking on a trail in a forest with her baby in a back carrier during the autumn season. Filtered to give retro, faded look.
Credit: Getty Images

"We do not have any evidence indicating that Jonathan, Ellen, or Miju ingested any of that water," Mariposa County Sheriff-Coroner Jeremy Briese said Thursday at a press conference. "And we also know that there has not been any reported deaths of humans connected to Anatoxin-a."

Briese also noted that local temperatures were between 107 and 109 degrees when the family died. The trail the family was on included steep and somewhat difficult terrain. While the family had an 85-ounce water bladder backpack, snacks, and a bottle of baby formula, Briese said the water bladder was empty when the bodies were found.

"This is a real tragedy," Briese said. "An unfortunate and tragic event due to the weather."

The sad news raises some obvious questions about hyperthermia, dehydration, and how to stay safe when you're in extreme temperatures. Here's what you need to know.

What is hyperthermia?

Hyperthermia is an abnormally high body temperature that happens when the body can't cool itself enough when temperatures are high, according to the National Institutes of Health (NIH). It also encompasses heat-related illnesses like heat fatigue, heat syncope, heat cramps, heat exhaustion, and heat stroke.

Heat stroke is a life-threatening form of hyperthermia—it happens when the body is overwhelmed by heat and unable to control its temperature, which usually increases to above 104 degrees Fahrenheit, the NIH says.

What is dehydration?

Dehydration is a condition that happens when you lose too much fluid from the body, according to the US National Library of Medicine. It can strike when you lose more fluids than you take in and your body doesn't have enough fluids to work properly.

Symptoms of dehydration include:

  • Extreme thirst
  • Dry mouth
  • Less urine and sweat than usual
  • Dark-colored urine
  • Dry skin
  • Fatigue
  • Dizziness

There is a range with dehydration, from mild to severe, the US National Library of Medicine says.

Why are hyperthermia and dehydration so dangerous?

The two conditions can be related. "Hyperthermia can be very dangerous because most of our body's biochemical and physical functioning work at a specific range of temperatures," Justin Johnson, MD, a critical care and emergency medicine physician who sees patients at Mercy Medical Center in Baltimore, MD, tells Health. "Outside of those ranges, it doesn't allow our vital functions to perform properly. For example, our brain at elevated temperatures becomes confused and sluggish. This can affect our ability to seek cooler climates."

Organs can "start to fail" at temperatures that are too high, Lewis Nelson, MD, chair of emergency medicine at Rutgers New Jersey Medical School, tells Health. "The primary reasons for the development of hyperthermia are that your body generates too much heat, such as while exercising or when intoxicated with a stimulant, or you fail to cool properly, such as when dehydration limits sweating," he explains. "The greatest overall risk for hyperthermia is, not surprisingly, being active in a hot environment, particularly if humid."

If hyperthermia progresses, it can cause the brain, liver, kidney, and muscles to fail, Dr. Nelson says, adding, "no organ is immune to high body temperatures."

With dehydration, hot environments cause you to sweat more, and when you're not replacing those fluids fast enough, you end up losing more water, Dr. Nelson explains. "Like hyperthermia, severe dehydration can cause organ failure and lead to death," he says. "It is typically slow to develop over days, unlike hyperthermic death, which can occur over a few hours of extreme exposure."

Warning signs to look for

If you're planning to be out in high temperatures, experts say it's important to be aware of the warning signs of hyperthermia and dehydration.

According to the NIH, severe symptoms of hyperthermia can include:

  • Mental confusion
  • Strong rapid pulse
  • Lack of sweating
  • Dry, flushed skin
  • Faintness
  • Staggering
  • Coma

And, per the US National Library of Medicine, severe symptoms of dehydration include:

  • Confusion
  • Fainting
  • Lack of urination
  • Rapid heartbeat
  • Rapid breathing
  • Shock

If you or someone you're with develops any of these symptoms, try to seek shelter in a cool spot immediately and get medical attention.

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