So how much is too much?

Jay Moulton is an engineer and the owner of RF Exposure Lab. He explained that the federal government regulates radiation from wireless devices such as cell phones.

"There is a limit on the amount of RF energy -- radio frequency -- that the human body is allowed to absorb," said Moulton.

It’s Moulton’s job to figure out how much radiation phones emit. His San Marcos testing lab is one of the few in the country that measures the radiation values of wireless devices. That measurement is called the specific absorption rate (SAR).

He showed 10News how the measurements are made by testing an iPhone.

Inside a copper-lined room was a machine with a plastic head-shaped mold. The mold was filled with liquid to simulate brain tissue.

Moulton secured the phone against the cheek and ear of the plastic head.

"We'll click start and the robot will pick up and start taking that measurement all around the phone," said Moulton as a robotic arm lowered a probe into the liquid.

After 20 minutes, a computer hooked up to the machine illustrated the results with graphs and color diagrams. They showed how the radiation from the phone was absorbed into the simulated brain. The values for this demonstration were within what the government considers safe.

The maximum radiation SAR allowed is 1.6 watts per kilogram.

"The limit has been determined based on 50 times below the level it takes to raise the tissue 1 degree Celsius," explained Moulton.

While all phones sold must meet this government safety standard, different phones have lower or higher radiation values. Not everyone believes these different radiation levels are a cause for concern.

TechAmerica, a technology trade association, said it is not a measure of safety. A representative wrote 10News, "The weight of the evidence does not suggest that any phone that meets that standard will have a greater impact on health than any other phone that meets that standard, even if the first has a higher SAR rating."

Olga Naidenko said more research is needed. She is a senior scientist with Environmental Working Group.

"But what we do know is less exposure which means less radiation is probably safer and better than more exposure," said Naidenko.

She cited studies that indicated health risks.

"Longer than 10 years of cell phone use increases the risk of two kinds of brain tumors and salivary gland tumors. Which makes sense if you think where we hold our phone," said Naidenko.

How children are affected by cell phones concerned her.

"The research has shown that the brains of children would absorb at least twice more radiation compared to adults," said Naidenko.

The President’s Cancer Panel addressed these same concerns in an April report about reducing environmental cancer risks.

But TechAmerica stressed current research does not indicate "low levels of RF energy emitted by cell phones and/or wireless devices pose health hazards."

Naidenko said she is not recommending people stop using cell phones.

"We all really need them," she said. "It's just we can use them safer."

State Sen. Mark Leno agreed and pointed out the FCC website includes a tip for consumers to buy a wireless device with a lower radiation SAR.

"But if a consumer goes to a store and the salesperson doesn't even know what you're talking about, how can you follow that FCC recommendation?" asked Leno.

Leno proposed a law requiring the cell phone radiation value be displayed next to its selling price. That bill, SB 1212, is awaiting a vote on the Senate floor.

In the meantime, Environmental Working Group created a database of cell phone radiation values. They took on the project because finding the information on the FCC website was not easy.

"It’s not a very straightforward way," said Naidenko of the information on the FCC website.

Some practical advice for reducing radiation exposure includes texting instead of calling, using a headset, and keeping an active phone away from the head and body as much as possible.