An Environment Ministry panel compiled a proposal Friday to tighten restrictions on exhaust emissions from two-wheeled motor vehicles.

The restrictions to be proposed by the Central Environment Council will aim to reduce by between 50 percent and 85 percent emissions of hydrocarbons, nitrogen oxide and carbon monoxide in exhaust from motorbikes and mopeds sold after 2006.

The council will submit the proposal to Environment Minister Shunichi Suzuki, possibly in June, and the new standards, based on the Air Pollution Law, will be announced officially next year, panel members said.

The proposed restrictions would reinforce rules considered the most stringent in the world since their imposition in 1998, they said.

The move comes as major technological advances that have paved the way for reductions in emissions from four-wheel vehicles have made it possible for tighter controls on motorbikes, they explained.

The new limits would be applied in 2006 for motorbikes with engines of less than 50cc and in 2007 for engines from 50cc to 125cc.

For both categories, the new limits would reduce currently allowed emission levels of hydrocarbons by 75 percent, nitrogen oxide emissions by 50 percent and carbon monoxide by 85 percent.

As a result, the currently permissible emissions of hydrocarbons would be slashed to 0.5 gram per km, while nitrogen oxide emission would be reduced to 0.15 gram per km. Emissions of carbon monoxide would be cut to 2 grams per km.

For motorcycles with engines between 125cc and 250cc, the new restrictions would be applied in 2006, and in 2007 for those above 250cc, with hydrocarbon emissions to be cut by 85 percent to 0.3 gram per km and nitrogen oxide emissions by 50 percent to 0.15 gram.

Hydrocarbon emissions are caused by imperfect combustion of gasoline and become suspended particulate matter in the atmosphere, a cause of asthma and other respiratory diseases.

Motorcycles emit 10 times more hydrocarbons than passenger cars, allowing for engine size.