WASHINGTON – Temperatures are rising — and so are reports of infants and toddlers dying from being trapped in 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 let kids 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.

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.