Mobile phone companies are fighting back after San Francisco passed new requirements for cellphone retailers to warn customers about limiting their exposure to radiation.

Among the requirements that kick in this week, stores will have to display "informational posters" explaining that cellphones emit radio frequency energy that's absorbed by the head and body.

The posters will also have to explain how to reduce that exposure risk — by doing things like using a headset or speaker phone, texting instead of talking, and limiting use by children.

The CTIA — a trade group representing cellphone companies — is filing a lawsuit in federal court, claiming the new law violates cellphone companies' First Amendment Rights of freedom of speech.

The cellphone industry is arguing that the legislation makes cellphone makers say something they don't want to say.

It's the first cellphone ordinance in the United States, created after numerous studies examined by the International Agency for Research on Cancer led the organization to classify cellphones as possibly carcinogenic to humans.

Leading epidemiologists who have studied how cellphone radiation is absorbed by the body have also recommended that the public be warned of potential risks, particularly with children.

On Tuesday, Health Canada said that while "at present, the scientific evidence is far from conclusive and more research is required" about any cellphone links to cancer, it issued warnings about cellphone usage.

The new regulations are somewhat watered down, after earlier proposed regulations were stalled due to a different lawsuit by the cellphone industry.

Phone companies argued that phones should not have to display their SAR rating — the Specific Absorption Rate of radiation emitted by each phone.

The industry claims that since all levels fall within a range considered safe by federal health regulators, providing that information to customers might make them think one phone is "safer" than another.

Under the new regulations, SAR ratings do not have to be posted, but should be listed in the phone's manual — usually located inside the cellphone box, and available to read only once the purchase has been made.

When Marketplace told people about SAR ratings last year, most said they wanted that information readily available, to make their own decisions.

In Canada, there are no mandatory requirements for retailers or cellphone manufacturers to alert customers of any potential concerns regarding cellphone radiation.

When Marketplace asked the Canadian Wireless Telecommunications Association about San Francisco's new regulations, spokesperson Marc Choma said, "All wireless devices sold in Canada are deemed to be safe by the federal government and no public safety warnings at point of sale are required or warranted."

Part of the controversy revolves around how cellphones are tested for radiation levels.

Few people know that cellphone guidelines are based on tests primarily done by cellphone manufacturers.

Scientists perform tests on a "dummy," representing the "average user" — a mould of a human head and torso, filled with a fluid mixture designed to simulate the electrical properties of human tissue.

But critics argue that the dummy is not representative of the average person. It weighs 200 pounds, is 6-2" tall, and only makes six-minute phone calls. The test is also done with the cellphone held at least one inch from the ear, a practice seldom employed by people making a cellphone call.