WASHINGTON (AP) _ Paper cartons can contaminate milk with cancer-causing dioxin, but at levels too low to pose a health hazard, the Food and Drug Administration said Friday.

The FDA said the paper industry was changing its manufacturing methods and until the changes are complete, ''the very low levels do not pose a threat.''

''Milk is safe to drink,'' FDA Commissioner Frank Young said. ''But because we have the means to virtually eliminate even this low level of dioxin, it is prudent to do so.''

The FDA said it will monitor the efforts of the paper industry to modify production of bleached paper and paperboard so they contribute virtually no dioxin to milk.

Dioxin and a related but less potent compound called TCDF result from reactions involving chlorine, which is used to make paper white and to help break down lignin, a natural material that binds the cellulose in wood.

Red Cavaney, president of the American Paper Institute, said about 75 percent of the paperboard for milk cartons is being manufactured under the revised process. The rest is expected to be brought under the new processing techniques within the next 18 months, he said.

Processing methods also are being revised for paper used to package other foods.

The agency said its tests detected dioxin in milk at levels far below 1 part per trillion in chlorine-bleached, polyethylene-coated paper cartons.

Of the 15 half-pint samples, dioxin levels in four of them ranged from 0.02 ppt to 0.07 ppt. None was found in the other 11 samples, the FDA said.

Three samples contained dioxin as well as TCDF. This substance was detected at levels of 0.14 ppt to 0.62 ppt in seven samples and not at all in the other eight.

Before testing, the milk samples were stored refrigerated for two weeks, the shelf life for milk, to allow for the maximum seepage of dioxin into the milk.

The FDA estimated the lifetime cancer risk would be less than one in a million from consuming milk packaged in paper cartons over the next three years to five years, even if all milk contained 0.01 ppt of dioxin and 0.05 ppt of TCDF.

Canada and New Zealand also have found their milk products to be contaminated with very low levels of dioxin, the FDA said.

The FDA's study stems from a national survey of dioxin begun by the Environmental Protection Agency in 1983, an offshoot of which found dioxin downstream from some paper mills.