Study of Excessive Temperatures in Enclosed Vehicles
(PDF version)
Jan Null, CCM ^1,2
June 2003

BACKGROUND
Every year dozens of children tragically die due to hyperthermia (heat stroke) after being left unattended in cars, trucks and vans.  Over the past five years in the United States 160 deaths of this type have been documented  (Kids and Cars and 4 R Kids Sake^TM, 2003).  [For the current year see 2003 Hyperthermia Fatalities (Children in Vehicles)].  Hundreds of other children left in similar situations suffer great bodily harm and these numbers do not include similar consequences to infirm adults or animals.

OBJECTIVE
This study quantifies vehicle temperatures and temperature changes with time under a variety of meteorological circumstances.

STUDY DESIGN
Temperature sensors were placed two types of vehicles on sunny days with ambient temperatures ranging from 70 F (21C) to 100 F (38C).  The initial interior temperature was regulated to equal the ambient temperature.  The vehicles were in full sun but the sensors in the vehicles were not subject to direct sunlight and were not in direct contact with any interior surface.  Except as noted in the results, the trials were conducted in a mid-sized dark blue sedan with medium gray interior and all of the windows were fully closed.  Using wireless temperature sensors the ambient outside and the inside vehicle temperatures were sampled every five minutes for at least one hour.  A secondary vehicle (a white mini-van with light gray interior) was used for some tests.  Neither vehicle had tinted windows.

TRIAL SUMMARY
Between May 16, 2002 and Aug 8, 2002 twenty-one trials were conducted on 15 days.  Table 1 summarizes the ambient and inside vehicle temperatures on the primary test vehicle with the windows fully closed.  Table 2 compares the temperature in the primary test vehicle on three occasions when the windows were fully closed and then when they were “cracked” open approximately 1.5 inches.  Table 3 is a summary of two additional test cases using the primary vehicle and the secondary vehicle.

METHODS AND PROCEDURES

Vehicle selection. The primary test vehicle was a 2000 Honda Accord.  The exterior color is dark blue and the interior is a medium gray.  The windows are not tinted.  The secondary vehicle in the study is a 1997 Honda Odyssey minivan which is white in color with a light gray interior and without tinted windows.

Measurements. Ambient temperatures were recorded with a Davis Instruments Vantage Pro Sensor Suite.  Wireless temperature sensors were placed in the test vehicles which transmitted to and were archived in the Vantage Pro base station. The temperature sensors had a resolution of 1 F (0.6C) and an accuracy of +/- 1 F (0.6C).  All readings were taken out of direct sunlight.

Experimental procedures. Trials were conducted on a number of cloud-free days with varying ambient temperatures.  The vehicles were parked facing at approximately a 45 degree angle away from the sun to minimize direct sunlight through the windshield.

Results
In the sixteen cases with the windows fully closed, the average temperature rise was 19 F (10.6C) in the first 10 minutes or 82% of the total 60 minute rise.  After 20 minutes the average rise was 29 F (16.1C) or 89% of the 60 minute total.  At the 30 minute mark the average rise was 33 F (18.3C) or 93% of the 60 minute total rise.  The average rise after 60 minutes was 43 F (24C) degrees.  In trials that exceeded 60 minutes the interior vehicle readings stabilized. (see Figures 1 & 2)

Figure 1.

Figure 2.

On the three occasions with the windows “cracked” open the average 10 minute rise was 17 F (9.4C), then 21 F (11.7C) after 20 minutes, 24 F (13.3C) after 30 minutes and the 60 minutes average rise of 38 F (21.1C).

An additional two trials were conducted with simultaneous readings in the primary test vehicle and the secondary vehicle.  (see Figure 3)  The average 10 minute rise in the primary test vehicle was 19 F (10.6C) compared to 10 F (5.6C) in the secondary vehicle.  After 20 minutes the respective average rises were 29 F (16.1C) and 18 F (10.0C), and after 30 minutes they were 34 F (18.9C) and 24 F (13.3C).  The average one hour rises were 42 F (23.3C) and 34 F (18.9C) respectively.

Figure 3.

Further studies should also be undertaken to delineate the effect of other factors on a vehicle’s interior temperature.  These variables include vehicles of different exterior and interior colors and materials as well as window tinting and window coverings.

In addition to children further studies should look at the effects of heat stress on adults and also pets.

CONCLUSIONS
When temperatures exceed 80 F (26.7C) potentially lethal temperatures of 105 F (40.6C) plus can be reached in less than 20 minutes and when they exceed 88 F (31.1C) lethal readings can be reached in 10 minutes or less. Only minor mitigation achieved by “cracking” the windows or having a vehicle of larger size or a lighter color.

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