Mobile phone manufacturers, responding to consumer and regulatory pressure, are using fewer toxic substances in their products, researchers in the United States said Wednesday.

The Motorola Citrus, Apple iPhone 4S and LE Remarq emerged as the least toxic cell phones in a study of 36 different models that have come onto the market during the past five years, the Michigan-based Ecology Center said.

The newly released iPhone 5 ranked fifth, and its arch rival Samsung Galaxy S III ninth, while the iPhone 2G -- the first in the top-selling smartphone series, released in 2007 -- was found to contain the most toxic materials.

"The takeaway is that mobile phones are chemically intensive, and full of chemical hazards, but they've been getting a lot better," Jeff Gearhart, research director at the Ecology Center, told AFP in a telephone interview.

In a statement, the Ecology Center explained that "every phone sampled in this study contained at least one of following hazardous chemicals: lead, bromine, chlorine, mercury and cadmium."

Such dangerous substances can pollute at any stage of a product's life cycle, from the moment they are extracted from the ground to the time a cellphone is assembled and the day it is thrown out, it said.

"Consumer interest in healthier products is driving companies to design and produce healthier products," said Gearhart, who also cited tougher controls in Europe and Asia over the hazardous materials used in consumer electronics.

The Ecology Center -- which posted its findings on its HealthyStuff.org website -- has previously looked into the extent of toxins in automobiles, children's car seats, jewelry, garden hoses and Halloween products.