Chemicals from plastic floating in the ocean pose a likely threat to marine life, a new study says.

University of California, Davis, researchers found that plastics soak up contaminants in seawater. Marine creatures that ingest plastics are threatened by both the material and the pollutants, the study says.

"It surprised us that even after a year, some plastics would continue to take up contaminants," said doctoral student Chelsea Rochman, who led the study. "As the plastic continues to degrade, it's potentially getting more and more hazardous to organisms as they absorb more and more contaminants."

The study represents the first controlled, long-term evaluation of how much the five most common plastics absorb contaminants, UC Davis said. The findings were published in the journal Environmental Science & Technology.

Plastics examined by the research:

• Polyethylene terephthalate, or PET, used in water bottles.

• High-density polyethylene, HDPE, found in detergent bottles.

• Polyvinyl chloride, PVC, turned into clear food packaging.

• Low-density polyethylene, LDPE, found in plastic shopping bags.

• Polypropylene, PP, used for yogurt containers and bottle caps.

Investigators filled mesh bags with pellets of the five plastic types and tied those to docks at five locations in the San Diego Bay. They retrieved the bags several times over a year and measured how much the plastics had soaked up pollutants in the water.

Researchers initially thought that the pellets would stop absorbing toxics after several months. They found, however that HDPE and LDPE continued taking in contaminants throughout the 12-month probe, UC Davis said. The analysis estimated that at one study site, it would have taken 44 months for HDPE and 19 months for LDPE to stop soaking in substances.

The most commonly produced plastics, polyethylene and polypropylene, "absorbed much greater concentrations of contaminants than PET or PVC," Rochman said.

That means that products made from HDPE, LDPE and PP might present more of a chemical risk to marine creatures, the study said.

HDPE, LDPE and PP constituted 62 percent of all plastics produced globally in 2007, the study says. PVC and PET made up 19 percent and 7 percent respectively.