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Temperature vs. time
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Picture this; its a hot summers day, you got up early to go to the park, and, having spent a few hours there, are ready to drive home. You open your car and immediately regret your decision - your car is now roughly the temperature of the sun! The seats have become molten lava and don't even think about touching the steering wheel. Many experiments have shown that even on a cooler summer day, temperatures inside a closed vehicle can quickly exceed 125°F (about 52°C). It seems that cars are built to turn into a hot oven, and, as you will see later, we mean it literally! All your stuff, including electronic devices, medicine, or food, could end up getting permanently damaged at these temperatures.

But, most importantly, by leaving pets or people in the parked car, you put their lives at grave risk. Some statistics estimate that about 40 children die per year due to being left in hot cars, and more than half of them were under two years old. Last year (2018) was the deadliest year in the past 20 years in the US (53 children died in hot cars), and there have already been 34 heatstroke deaths in 2019. Take a moment and explore this topic a little more with the hot car calculator, so that you become well aware of heat hazards!

Child hyperthermia deaths statistics

Heatstroke hazards in closed vehicles

Global warming is a fact, and we now have to deal with the health consequences of increased temperatures. We should pay special attention to babies, children, pregnant women, and the elderly as these individuals are the most vulnerable in hot weather, according to the Department of Health of Government of Western Australia publication Health effects of heat – who is most at risk?

Jan Null is a Certified Consulting Meteorologist that continuously monitors reports on child vehicular heatstroke deaths in the U.S.. It turns out that between 1998 and August 2019, 831 children have died from vehicular heatstroke, and many more have been put in a life-threatening state. It is worth noting that only 18.9% of these kids were left knowingly by a caregiver. Most of them (54%) were just forgotten or gained access on their own (26.3%). Below you will find some descriptions of death certificates from Jan Null's site. Don't these situations sound like something that can happen to anyone? Are you sure you will never let this happen to your children?

  • 16th July 2019, Richmond in Virginia, temperatures in Richmond were in the low 90s (about 32°C): 10-months old baby girl was left in the car while her mother went grocery shopping. The car was then driven to a home in South Richmond. That's where emergency crews responded just before 2 p.m. The child, who was in cardiac arrest, died on the way to the hospital.
  • 2nd July 2019, Galveston in Texas, air temperature: 93°F (34°C) - "Around 11 a.m., a parent arrived for work at Mexican restaurant, and left a child in the car. The parent returned to the car around 4 p.m., and found the toddler to be unresponsive in a hot car outside of restaurant." - the 1-year-old boy died.
  • 8th June 2019, Butler County in Kansas: 86°F (30°C) - "When a mother arrived with her daughter to her home, she felt drowsy and went inside to take a nap. After realizing she left the baby behind, the mother rushed back out to the vehicle and found the child unresponsive and not breathing." - the 3-month-old girl died.
  • 22nd May 2019, Blythewood in South Carolina: 88°F (31°C) - "Zion was in town visiting family members when his family started searching for him after his mother noticed he was not present in home. He was later found unresponsive in the back of the family's car. The family believes he walked out of the side door and accidentally got locked inside the car." - the 4-year old boy died.
  • 6th May 2019, Lakewood Township in New Jersey: 69°F (21°C) - "Girl's mother tried to take her to daycare, but the toddler didn't want to go in, so they drove home together. At that point, there was a miscommunication about whether the toddler’s mother or father would bring her in from the car. She has been left in the car." - almost 2-year old toddler died.

There are many more examples of similar tragedies. The danger of a hot car is real, and should never be underestimated, even if there is "only" around 70°F (20°C), such as in the last case. If you want to educate yourself on how to keep your child safe, you should check the Hot Car Safety Tips for Parents, prepared by certified pediatrician Vincent Iannelli which can be found at verywellfamily.com. Make it a habit to always think of your children and where they are, and prevent them from getting into a vehicle by themselves.

Child hyperthermia deaths statistics

Interior temperatures - turning a car into an oven

You can find many works that consider the severity of leaving children and pets in a stationary vehicle on a hot summers day. All of them unanimously agree the temperature in the cabin of the car can become 70°F (20°C) hotter than the ambient temperature outside, even on relatively cool and cloudy days. Andrew Grundstein from the University of Georgia is a world-renown expert in the field of climate and human health, with a large number of comprehensive publications. In one of them, he thoroughly studied the Maximum vehicle cabin temperatures under different meteorological conditions, and developed two simple predictive models (based on solar radiation and sky cloud cover) that can be used interchangeably.

To understand how dangerous an increased cabin temperatures could be, let's take a look at the terminology of the National Weather Service (NWS), an organization of the United States that provides weather forecasts and warnings. The NWS glossary issues two warnings depending on the heat index value (a combination of air temperature and relative humidity):

  • Heat advisory when the heat index is between 105°F (40.5 °C) and 115 °F (46°C) for less than 3 hours. At this point, people can be affected by heat, often leading to an increased number of emergency room visits.
  • Excessive heat warning is issued if the time spent in the 105°F - 115 °F (40.5 °C - 46°C) range is longer than 3 hours or heat index is greater than 115 °F (46°C) for any period of time. This is when people are in severe danger, and mortality begins to increase rapidly.

Are you still not sure about this threat? Just watch the following video demonstrating a car turning into the oven, literally. Australian chef Matt Moran overcooked a delicious lamb loin in little over hour and a half using only his car. Now, try to imagine that there is a kid or a dog trapped inside that vehicle!

Temperature vs. time - do open windows help?

So, why does the interior of a car get hotter than its surroundings? In simple terms, it is because of the greenhouse effect occurring in the vehicle, the same effect that is the main cause of global warming. Car windows act similarly to the Earth's atmosphere; they are almost transparent to light radiation (which has short wavelengths), but prevent thermal radiation (long wavelengths) from leaving. Therefore, solar radiation goes through the windows and heats the interior elements of the car, e.g., dashboard, seats, or floor. Those elements then emit thermal radiation, but it is blocked by the car windows when it tries to escape the car. This traps the heat radiation in the vehicle, causing the cabin temperature to increase dramatically.

Greenhouse effect

This greenhouse effect was analyzed experimentally during summer by Andrew Grundstein in the paper Quantifying the Heat-Related Hazard for Children in Motor Vehicles. With the collected data and simulations, he provided a representative table of temperature change rates inside a vehicle on days with various ambient air temperatures. Moreover, the Royal Automobile Club of Queensland Limited (RACQ), conducted several tests to measure the interior temperatures of parked cars in a Cars Survey. They concluded:

  • There is no safe situation to leave children unattended in a vehicle, even for a short amount of time.
  • Interior of the cars with dark paint color heats up faster compared to light colors, but only slightly.
  • Sunshades and windows tinting may slightly slow the rate of temperature rise, but final interior temperatures remain almost the same.
  • Leaving the partly open windows may reduce the temperature in a car.

Be aware that the last point doesn't mean it is safe to leave a living thing inside a parked vehicle. The rate of increase in the cabins temperature reduced, but it is still quite rapid. Use hot car calculator and find it out by yourself! So, leaving your windows open is not safe for a long period of time, and also isn't great for security.

Heat illness

When you spend some time in a high temperature, your bodies thermoregulatory efficiency decreases, and your body temperature starts to rise. We call this state hyperthermia; a body absorbs more heat than it dissipates. It is especially dangerous when body temperature is higher than 40 °C (104 °F). Such a situation is known medically as heat illness, which may proceed as:

  • heatstroke (the most dangerous)
  • heat cramps
  • heat rash
  • heat syncope
  • and others.

Typical symptoms of heatstroke include dizziness, headaches, confusion, sleepiness, seizures, and the absence of sweating. Treating heatstroke is focused on rapid mechanical cooling of the patient and intravenous dosages of fluid. Patients also receive drugs to treat seizures as well as painkillers. Heatstroke is a serious medical condition which can result in many complications and has a mortality rate of about 40%.

It's difficult to predict when heatstroke will strike because it involves many factors like person's age, weight and health conditions. Babies and young children are especially susceptible to high temperatures, and consequently to hyperthermia. Physicians at Children’s Hospital say that a child’s body can heat up three to five times faster than an adult’s, and when the body’s temperature reaches 104 degrees, the internal organs begin to shut down.

Why are children more vulnerable to hyperthermia? There are many reasons for that, here is the list of the most important ones according to UpToDate:

  • Heat production – Children produce more metabolic heat per kilogram of body weight because they have a higher basal metabolic rate than adults

  • Body surface area – Younger children have a higher body surface area to mass ratio, resulting in a greater rate of heat absorption in hot environments

  • Blood circulation – Children have a smaller absolute blood volume, which limits the potential of blood-borne heat transfer from the body core to the body surface, where heat is dissipated

  • Sweat production – Children have a lower rate of sweating than adults as a result of a lower sweat rate per gland and a higher body temperature at which they begin sweating

  • Fluid replenishment – If not appropriately supervised, children are more likely to inadequately replenish fluid losses during prolonged exercise

  • Acclimatization – Children achieve adaptations to a hot environment more slowly than adults

In the research paper, Evaluating the impact of solar radiation on pediatric heat balance within enclosed, hot vehicles, authors assessed core temperature rise of a toddler using specific heat capacity of a 13.4 kg (29.5 lbs) boy. They concluded that the core temperature of an infant enclosed in a car might change from 36.8 °C (98.2 °F) to lethal 40 °C (104 °F) in only one hour!

Picture of sleeping child

It is worth to note here that some regions have Good Samaritan Laws giving you the immunity if you act to protect the life of a child inside of a hot vehicle. If you ever see a child left unattended in a car, don't hesitate and use force (e.g., break a window) to save a child from a heatstroke. You will avoid any legal consequences. This law might differ in various places around the world, so be sure to check whether this applies your country!

How to use the hot car calculator?

The hot car calculator is based on a few studies, already mentioned in this article. It allows you to estimate the temperature in a car and the expected body temperature of a child over time. If you still have any doubts about using it, follow the following instruction:

  1. Type in the outside air temperature - our calculator uses a model that is most accurate on warm days when the air temperature ranges from about 68°F (20°C) to 122°F (50°C).
  2. Select the way you wish to estimate maximum car interior temperature, it will determine the next steps. You can use calculations based on either solar radiation or on the sky cloud coverage.
  • If you chose cloud cover - decide how much of the sky is covered in clouds or assess it in percentages in the advanced mode (clear sky - 0%, and overcast - 100%).
  • If you chose solar radiation - select a day of the year and one of the capitals of countries around the world. We will calculate solar radiation theoretically. For more precise estimations, you can type in the latitude of your town by hand in the advanced mode (the latitude of a particular place can be easily found on the Internet).
  1. Decide whether your car color is light or dark and if your car has either closed or partly open windows.
  2. That's it! You can now scroll down to see two interactive graphs:
  • The blue curve shows the change in the cars internal air temperature.
  • The yellow-red curve represents the estimation of a 2-year old boy's body temperature.

Each of the curves has some special points highlighted above the graph. These include the National Weather Service (NWS) heat warning systems on the blue curve and health consequences of increased human body temperature on the yellow-red curve.

Don't forget about pets!

Dogs, cats, and other animals can't sweat like humans. They can only pant to lower their body temperature. However, panting with hot air gives no relief at all, and the risk of heatstroke rises very rapidly. Don't leave your pet in a car "just for a minute". What if your business would take a little longer time or you forget about it entirely? People for the Ethical Treatment of Animals (PETA) state that animals can sustain brain damage or even die from heatstroke in just 15 minutes. Unlike child deaths, these tragedies are often ignored by media, but, in fact, thousands of animals suffer the same painful fate every year. If you own an animal, think twice before deciding to take your pet for a ride that includes stops and errands.

Picture of enclosed dog in car

Unfortunately, if you see the locked pet, you cannot always act similarly to the case of a child and Good Samaritan Laws. The vehicle's owner might sue you for the destruction of personal property. Instead, follow the advice of personal injury lawyers Hupy and Abraham: call 911, try to find the owner of the vehicle and shade the animal through the window until authorities arrive.

Łukasz Białek, MD and Dominik Czernia, PhD candidate

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