WASHINGTON (AP)  Temperatures are rising and so are reports of infants and toddlers dying from being trapped inside sizzling automobiles.

A researcher says 18 children have died of hyperthermia since the beginning of the year, with eight deaths reported since June 13. That's the largest number of fatalities through the first half of a year since Jan Null, an adjunct professor of meteorology at San Francisco State University, began tracking the data in the late 1990s.

Government and safety experts are telling parents that they never should leave children in an unattended vehicle or allow kids to play in cars and trucks. Many of the recent cases have involved children who climbed inside an unlocked vehicle on a hot day and then couldn't get out.

"These really are good parents who love these kids who make a mistake that turns out to be fatal," said David Strickland, the administrator of the National Highway Traffic Safety Administration. The government's highway safety agency issued a consumer advisory this week that included a warning for parents not to leave children unattended in or near a vehicle.

Null, who has compiled data on the cases through media reports, said 37 children typically die each year from heat exhaustion in vehicles. A NHTSA report in June 2009, based on police reports, estimated that 27 children died in 2003-2004 from hyperthermia.

The deaths in June have caught the attention of safety advocates because July tends to be the most deadly month for children trapped in hot cars. With a week left in June, the number of deaths has topped the previous high of 17 fatalities from January to June 2009, according to Null's data.

In 2005, when Null counted 47 child hyperthermia fatalities, only 12 of the deaths occurred through the end of June.

Since 1998, Null has documented 463 child deaths involving heat exhaustion inside cars and trucks. Safety advocates said the deaths have been more prevalent since the mid-1990s when parent-drivers were required to put their children in the back seat, where they are safer in transit but more likely to be forgotten.

Six fatalities have been reported in Texas, including three in the past month, along with two deaths apiece in Tennessee and Missouri.

In Hineston, Ala., 2-year-old Hunter Iles was found dead on Monday in the front passenger seat of his family's car after playing with other children outside his home. Sheriff's officials told local media outlets that the child was discovered missing and later found unconscious in the vehicle. Temperatures were in the 90s that day.

In Portageville, Mo., 2-year-old twins Allannah and Alliya Larry were found dead in their grandmother's car on June 16 as temperatures pushed into the mid-90s. New Madrid County Sheriff Terry Stevens said the children apparently got into the unlocked car on their own and were locked inside the vehicle for two hours.

When investigators arrived, he said, the temperatures inside the car had surpassed 140 degrees.

Children are particularly vulnerable because they have difficulty escaping on their own and their respiratory and circulatory systems can't handle heat as well as adults.

Safety groups such as Kids and Cars and Safe Kids USA urge parents to check the back seat every time they exit the vehicle and to create a reminder system for themselves.

Some parents leave their cellphone or purse on the floor near the car seat to ensure they retrieve it along with the child. Others remind themselves by placing a stuffed animal in the car seat when the child isn't using the seat and putting the toy in the front seat when the child is tucked in the car seat.

Copyright 2010 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.