Image copyright AFP

Every few days in the US, there are local media reports about young children dying from heatstroke, or being rescued, after being left to bake in parked cars. Why does it happen so often?

With the click of an opening car door, at 15:15 on a warm Florida spring day, Reggie McKinnon's world suddenly collapsed.

"I heard someone screaming," he says. "Then I realised the screaming was coming from me. The rest is just a total blur."

For three hours, his 17-month-old daughter Payton had been left in his Ford Explorer, where she died of heatstroke as the temperature steadily rose far above the 24C (75F) temperature outside.

McKinnon had earlier taken her to a doctor for an ear examination and then driven back to work, forgetting to drop her off at nursery.

Since her death in March 2010, nearly 150 children have died across the US, 17 this year. Several parents face criminal charges of neglect or, in the high-profile case of a father in Georgia, a murder charge.

What makes the case of that man, Justin Harris, so unusual is that he is accused of intentionally leaving his toddler son, Cooper, in the back seat to die, a charge he denies.

Image copyright Reuters Image caption In Georgia, Justin Ross Harris is accused of deliberately leaving his son in the car

Image copyright AP Image caption Cooper Harris is buried in Tuscaloosa, Alabama

Despite the publicity of cases like the one involving baby Cooper, the number of tragedies has remained on average at about 38 deaths a year for the past decade, says Jan Null, a consulting meteorologist who records such cases.

He believes his figures, which are based on local media reports, mask the true scale because he gets calls from bereaved relatives whose loved ones never made the news.

If more parents realised that the interior of a parked car can heat up 10C in just 10 minutes, there would be fewer tragedies, says campaign group Safe Kids Worldwide.

When the human body reaches temperatures above 40C (104F) the organs are at risk, but children are vulnerable because their bodies heat up three to five times faster than an adult's.

Car death victims are usually under two but have been as old as five, and deaths have occurred in outside temperatures as low as 23C (73F). Every state in the US has a sad story to tell, although last year Texas and Florida accounted for a quarter of all of the deaths.

How laws differ around US

Most states deal with hot car incidents through neglect laws, but 19 states address it directly, including:

A 15-minute limit on leaving a young child in a car in Florida

A traffic violation in California for leaving a young child, six or below, alone in unsafe conditions

Leaving a child under seven alone when unsafe to do so or engine is running is a crime in Tennessee and Nevada

There have been high-profile cases in other parts of the world too, notably in Australia and in Israel, where three deaths last summer made headlines.

The numbers in northern Europe appear to be proportionally lower than in the US, but that's hardly surprising given the cooler climate than the southern US states. Between 2007-2009, there were 26 cases of heatstroke in France and Belgium, including seven fatalities, according to Child Safety Europe. The Royal Society for the Prevention of Accidents could recall no cases in the UK.

"I've seen numbers out of Europe and the total is not as high as in the US but there are not as many drivers or cars so it is very difficult to compare," says Null.

Cases in the US began to rise in the late 1990s, as children's car seats moved from the front to the back due to the dangers posed by airbags.

"At the same time, car seat manufacturers were saying that rear-facing seats were safer so not only were the children in the back seat but also not even visible," says Null, who believes the advent of mobile phones and more hectic lifestyles were also a factor.

Many readers may be incredulous that a parent could leave a child behind, saying some mistakes are inexcusable. The response to these kinds of cases is commonly vitriolic. But Kate Carr, the president of Safe Kids Worldwide, says there are three ways a child can find itself trapped.

The majority of cases happen unintentionally, she says, when the driver goes into autopilot.

"I've met many of these families. It can happen to anyone, it's not a story of bad people - they are some of the most upstanding citizens you would think it won't ever happen to." Sometimes, she says, it's just busy lives and the intrusion of technology in cars that distracts people.

The second way is children who get into cars on their own, and this accounts for about 30% of all cases, says Carr. And the third is when carers intentionally leave children in cars for what they think is a safe period of time, perhaps mistakenly thinking an open window will prevent tragedy.

There was condemnation and sympathy in equal measure for Shanesha Taylor, who last week avoided prosecution for leaving her two young sons alone in a hot car while she was at a job interview. They were rescued unharmed. She had argued she couldn't find a babysitter and was desperate for work.

Image copyright AP Image caption Gratitude is all I feel, said a relieved Shanesha Taylor

The deliberate cases often occur in public places and passers-by sometimes spot the danger and intervene. A five-year-old was pulled to safety through a rear car window after being left for more than an hour in plus-32C (90F) temperatures in Albuquerque, New Mexico.

There have been attempts to make reminders for drivers - from a coloured band invented by a 12-year-old boy, to apps which give reminders. A petition has been presented to the White House urging the president to provide funding for more research into life-saving technologies.

"There's not a technical solution that's foolproof so we have to focus on behaviour - never leave a child in a car, keep the car keys away from kids and put reminders in the front or back seat," says Carr.

Visual cues can be homemade, like a mobile phone, shoe, purse or briefcase placed in the back with the child.

Tips from a campaign group

Keep a teddy bear in the car seat when it's empty, then when you put your child in the seat, move the animal to the front seat

Put a shoe or mobile phone in the seat with your child

Put the car seat on the passenger side of the back seat

Look in the front and back seats when you lock the car

Ask your partner to call to check you dropped your child off at nursery

Source: Kids Safe Worldwide

Asked whether a terrible mistake should be a crime, she says, "What greater punishment could you have than the knowledge that you've accidentally killed your child?"

After Payton's death, McKinnon was arrested and charged with leaving a child unattended for longer than 15 minutes resulting in bodily harm.

Looking at five years behind bars, he made a plea deal and underwent probation and community service instead. He goes to parenting groups to tell them his story, keeping a promise to Payton that he would try to save other young lives.

Image copyright Other Image caption Payton's father Reggie thought she was at nursery but she was still in his car

Image copyright Other Image caption Since Payton's death in 2010, McKinnon has had two more children

He has had two more children since Payton's death and his wife Julia, a high school sweetheart, has remained supportive, he says.

"In over 65% of these cases the significant other leaves the relationship," he says. "Most end in divorce. My wife knows I would never intentionally harm my children and she's stood by my side from the moment of our horrible loss."

Four years on, McKinnon, 42, is still haunted by what happened but believes people need to know because it could be them.

"People out there want to crucify me for what I did. I was one of those people before it happened to me.

"I would ask: 'How could they forget their child? I would never do that. That only happens to people who are uneducated, drunk, drug addicts - not me.'"

Subscribe to the BBC News Magazine's email newsletter to get articles sent to your inbox