BAGHDAD — The Humvee's air conditioner conked out right after four U.S. soldiers clambered aboard for a run through some of Baghdad's most dangerous streets. The temperature was 115 — and without AC, it would quickly rise to 150 or higher inside the vehicle.

So 1st Lt. Robert Plummer and his men took the lesser of two unpleasant choices: They got out and walked.

"If you're out walking around, there's not much you can do but sweat a lot, wearing 80 pounds of gear," said Plummer, a 31-year-old Alabama man with the 12th Infantry Regiment.

As the U.S. military spends its fifth blistering summer in Iraq, the Army is evaluating new garments to help fight the dangerous combination of extreme heat and the heavy protection needed on the battlefield.

Cooler soldiers "carry out their mission longer. They're less likely to make mistakes because they're mentally more alert," said Walter Teal, an engineer at an Army lab in Natick, Mass., where some of the heat-busting concepts are designed and tested.

The research has been built around a simple idea: vests that cool soldiers. But at the blazing summer temperatures in Iraq, it becomes a rather complicated issue of weight versus reward.

The trick is trying to enhance the heat-busting power of the vests without making them too heavy or cumbersome.

The most commonly used cooling system is the Interceptor Ventilator Vest, developed by the Natick labs in conjunction with other government agencies and private contractors. The vests were first sent to the Marines last year, and about 30,000 are in use. Their aerated honeycomb fabric creates a gap between a soldier's body armor and his body, allowing air to circulate around his torso and back and evaporate perspiration.

Natick's next generation — called the Body Ventilation System — takes the Interceptor vest a step further.

It involves a battery-powered air blower attached to a soldier's body to circulate dry air beneath his body armor.

The Army ordered 2,500 blower vests and tried them out last year with soldiers in Baghdad and in Saddam Hussein's hometown of Tikrit, among other places. Response to the garment — which costs about $850 — was mixed, said Lt. Col. Bill Garland, a project manager at the Army's Rapid Equipping Force.

Some soldiers said the amount of cooling didn't justify the bulk and extra weight, he said. Also, the battery wasn't compatible with other Army equipment. But other soldiers wanted to keep wearing the vest, which has the advantage of being the only one of its type ready to be used in the field.

The feedback led to some improvements, said Craig Bandes, chief executive of Global Secure Corp., which makes the Body Ventilation System after having developed it in conjunction with the Natick labs.

He said the vest's battery was replaced, the motor made lighter and the noise reduced. Also, he said, the material was changed so that the vest can be placed closer to a soldier's body, increasing cooling.

The Army is now considering whether to stick with the blower vest or use other systems.

Staff Sgt. Kevin Nettnin, 25, of Van Buren, Ark., wondered if the extra weight — about 4 pounds for the blower — was worth it.

"One hundred twenty degrees is 120 degrees, and putting more stuff on us is not something I'd want to do," he said, relaxing in shorts and a T-shirt at a dusty U.S. outpost in southern Baghdad.

From May until September, temperatures of 100 degrees or more are routine in Baghdad and even higher in deserts and lowlands. About the only break is on the slopes of the northern mountains in the Kurdish autonomous zone — far from any major military action.

Troops manage with a bit of ingenuity. They carry ice water in special packs — and even prolong questioning of Iraqis when they find a cool house.

Soldiers stay near the constant buzz of air conditioners on bases, where diesel generators are also constantly rumbling. The national electricity grid is off far more than it's on.

In fact, American troops, including many who have done multiple tours, appear resigned to the heat. The best weapon, they say, is water — gallons of it. Get near a U.S. soldier in Iraq and you're near water. Cases of it in bottles are stacked everywhere, iced coolers rattle in the back of armored vehicles, base refrigerators never run out.

Other beverages will work, too.

As Plummer recounted the day the AC broke down, he leaned against a Humvee while gripping an energy drink — a bit of relief from the polluted Tigris River's stench, which wafted around him during a recent minesweeping operation.

Liquid plays a key role in yet another Natick product, but this one is still a few years away from completion.

It consists of a flak jacket filled with thin tubes circulating a mixture of mostly water with 10 percent propylene glycol (an organic compound used in hydrolic systems and other commercial products) and a trace of an anti-fungal agent common in swimming pools.

Such vests have been used on helicopters and some Humvees in the past couple of years. The developers hope they can eventually make them usable for troops outside their armored vehicles.

The unit that circulates the water promises three times as much cooling as the blower vest, but weighs about 6.5 pounds. That's down from 27 pounds when research started in the mid-1980s, but still far too heavy.

Shaving off those last ounces may take a couple of years, researchers said.

The gear already worn by the troops makes them feel as if it were 15 degrees hotter than the air temperature, said Sgt. Derek Remaley, a 25-year-old medic from Twin Falls, Idaho.

"You just deal with it," Remaley said in between treating Iraqi casualties after a gunbattle in Baghdad.

In helicopters and Humvees where the system is in place, crew members attach their vests to cooling units installed in the vehicles. The water circulates through the vests as the vehicles move.

Regardless of the high-tech cooling systems, Garland and Remaley said, soldiers quickly get used to the heat.

"You get acclimatized to it real quick," Remaley said. "When it gets hot, you don't stop what you're doing."

Kim Gamel reported from Baghdad and Jay Lindsay from Natick, Mass.