Dioxin levels in air throughout Japan improved in 1998, but levels of benzene -- a potent carcinogen found in automobile exhaust -- are still above government safety levels, according to the government's most comprehensive survey of harmful airborne chemicals.

The study, released Tuesday, was carried out by the Environment Agency and municipal governments to measure the concentrations of 19 harmful chemicals at hundreds of sites throughout the country.

Dioxin was lower on average than the year before, with an average value of 0.23 picograms per cubic meter. This year, only two of the 458 sites showed dioxin levels above the government's 0.8 picogram guideline, whereas 21 percent of the surveyed sites exceeded guidelines in 1997. A picogram is a trillionth of a gram.

In addition, 52 sites surveyed consecutively in both 1997 and 1998 for comparison purposes showed that the average dioxin concentration dropped from 0.56 to 0.31 picograms per cubic meter of air, the report says, adding that the highest recorded value was cut nearly in half, from 1.4 to 0.71 picograms.

While the dioxin news was welcome, data showed that benzene levels in the air exceeded the Environment Agency's standard of 3 micrograms per cubic meter of air at 135 sites, or 46 percent of those surveyed.

The highest figure recorded was 11 micrograms per cubic meter, with the national average coming to 3.3 micrograms.

Of all the sites surveyed, 458 were tested for dioxin and 292 for benzene. The sites were classified into three types: roadside, general or areas near suspected waste emitters.

Agency officials admit the benzene situation is "inappropriate," and say they plan to look at it in more depth when the Air Pollution Control Law comes up for review next April.