Science Made Simple: Understanding the Basics of Hot Car Deaths

Each year, 37 kids – not to mention hundreds of pets – die from heatstroke because they were left in a hot car.

It’s a senseless death: entirely driven from a lack of awareness and understanding of how hot cars can truly get in such a short period of time.

We spoke with meteorologist Jan Null, one of the leading experts in hot car deaths in the United States, to understand more about hot car deaths and what justice professionals need to understand in order to investigate them more effectively.

(This interview has been edited for length and clarity.)

Justice Clearinghouse Editors (JCH): Jan, tell us a little bit about you. You’re a climatologist/meteorologist – but you are one of the leading researchers/experts about hot car deaths in the US. How did you get involved in this field of work?

Jan Null: Back in 2001, on an 86-degree summer day, a 5-month old boy died in a hot car in San Jose, CA.  At that time I was asked by the media “how hot can it get in that car?” and the only study I could find was from 1993 in Louisiana and only looked at a single 93-degree day.

Out of scientific curiosity, I started casually tracking temperatures in my own vehicles and was startled at not only how hot the readings were but also how rapidly they rose.

In the summer of 2002, I did a controlled study looking at vehicle temperatures on 16 days with ambient temperatures ranging from 72 to 96 degrees. I started working with two Stanford University Hospital Emergency Medicine doctors to meld my data with the pediatric heat stroke information into an article that we published in peer-reviewed Pediatrics in 2005. Since its publication it became the “go to” article on the topic and is used worldwide.



JCH: What are the most important facts a prosecutor or law enforcement officer should remember when investigating or prosecuting a hot car death?  

Jan: The types of circumstances that lead to heatstroke deaths of children in vehicles fall into three categories.

JanNull - Circumstances for kids in car


The largest group (54%) are when a parent or another caregiver forgets a child and leaves them behind accidentally when exiting the vehicle. This has been referred to by some as “Forgotten Baby Syndrome.”

The second group, comprising 28% of the cases are when a child gains access to a vehicle on their own, as opposed to being put into the vehicle by a caregiver.

And the last category are instances where a parent or other caregiver make a conscious choice to leave the child in a vehicle while they undertake some other activity (e.g., an appointment, work, going to a racetrack, bar or casino, getting their hair done, etc.) The only documented case where a person has been convicted for leaving a child to intentionally do them harm was the Justin Ross Harris case in Georgia from 2014.



JCH: What misconceptions or misunderstandings do you see among law enforcement or prosecutors regarding hot car deaths?

Jan: Not just law enforcement and prosecutors, but also the general public, think that it has to be a hot day for a death or serious injury to happen in a vehicle. One of the important takeaways from my research has been the number of cases when the temperatures were not extreme. Of 694 cases between 1998 and 2016, 217 (42%) were on days when the temperature was less than 90 degrees. There were even 21 days (3%) then the mercury was less than 70.

Jan Null - Temps for Hot Car Deaths


The Pediatrics study, which looked at 16 days with temperatures ranging from 72 to 96 degrees showed a relatively straightforward relationship between the ambient (i.e., outside) and interior temperature. The average temperature rise in the first 10 minutes is 19 degrees and another 10 more degrees in the second 10 minutes. This means that two-thirds of the first hour’s temperature rise, which averages 43 degrees, occurs in the first 20 minutes!

Hot Car Average Temperature Rise
Graphic Credit: Jan Null


It should be further noted that these measurements are of the air not in direct sunlight. Measurements of surfaces in direct sunlight on a relatively mild 80-degree day can easily be in excess of 180 degrees. Consequently, in cases where a child is left in a car for multiple hours, it is possible they are in direct sunlight which greatly exacerbates the risk of heatstroke.


JCH: When looking at the data of over 700 deaths, what observations do you have that our membership might find interesting?

Jan: One of the factors I am often asked about is the mitigation to a vehicle’s temperature by “cracking its windows. This was something I looked at in my initial 2005 Pediatrics study when I measured the interior temperatures with all four windows open a couple of inches.  Overall, the differences were only about two degrees cooler, but with interior temperatures of 130 degrees plus it’s insignificant in terms of survivability.

I am also frequently asked about the effect of humidity and the “heat index”.  While outside relative humidity data is readily available, the humidity inside a vehicle can be much higher due to perspiration and respiration.


JCH: While a large percentage of our members are part of the law enforcement community, we have members from across the justice arena, particularly prosecutors. What resources or tools do you have on that might help them with their work?

Jan: Most jurisdictions, fortunately, probably have not seen a heatstroke death of a child in a vehicle. The information on is designed as a tool to be used to answer initial questions about how hot cars can get plus about how often these deaths occur and under what circumstances. Additionally, I have developed a Hot Vehicle Incident Checklist which may be a useful starting point when there is an investigation.


To watch the Investigating & Prosecuting Hot Car Deaths webinar, click here.


Additional Resources
4 years ago
Prosecuting & Investigating Hot Car Deaths: An Interview with Hilary Weinberg and Joshua Clark
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