'Tis the season for 110-degree days, excessive-heat warnings and trolling the parking lot for a shady spot. But new research suggests those in the shade might not have it made after all.

Pediatric vehicular hyperthermia (PVH), the heatstroke that kills 30 to 60 children in cars each year, has less to do with cabin temperature than with a child's core temperature. With this in mind, a new study has examined heat effects on a simulated 2-year-old boy.

The research appeared online on May 23 in the journal Temperature.

"It's much more difficult for a child's system to cool and sweat to be able to get evaporative cooling," said state climatologist Nancy Selover of Arizona State University, who co-authored the study.

As input, the model used real summer heat data from inside three pairs of Tempe cars: two sedans, two economy cars and two minivans. One of each pair was parked in the shade; the other, in the sun.

The model included factors like absorbed radiation, metabolic body heat and heat lost or gained through conduction, air movement and sweat.

In shade, the boy's core temp topped 100 Fahrenheit (38.2 C) within an hour and a lethal 104 (40.0 C) within 2.4 hours.

In sun, the same temp took only 1.4 hours to reach.

Previous studies have shown that rates of increase in core temperature vary by body size, sex and age.

Pound for pound, children's bodies produce more heat than adult bodies and experience faster rise in core temperature. Children are also less efficient at cooling through sweating.