The fact that Maine policy-makers are scheduled to debate today whether a rebate for the purchase of a low-polluting car is as effective at cleaning the air as, say, an incentive for getting clunkers off the road is a positive sign in the fight against pollution. It means that Maine properly sees automobiles as a significant source of air pollution and is looking for ways to correct it.

LD 2182 would offer rebates of between $500 and $2,000 to purchasers of cars that are fuel efficient and low polluting. The cleaner the car, the higher the rebate. The idea, which has gotten a big push from the Natural Resources Council of Maine, makes sense for several reasons. Not only does it reward drivers who choose cars that pollute less, it draws attention to the availability of these new vehicles and, if effective, would put a lot more of them in Maine. That, in turn, would both create a market for the manufacturers of these vehicles and eventually add a more efficient fleet to the used-car business.

Ronald Severance, air-quality specialist for the Department of Environmental Protection, recently suggested that paying people to get that 15-year-old Monte Carlo and other older cars off the road would be a more effective use of the rebate money, which would be raised by adding a surcharge to the excise tax on cars — $5 for new cars, $3 for used. Measured in pounds of pollutants over the short term, Mr. Severance may well have a point. The long-term benefit of having more of these cleaner cars replace the typical vehicle in Maine, however, might change that equation.

Neither proposal is perfect. The new-car rebate doesn’t require that you get rid of your old car, a clunker buyout doesn’t guarantee that a driver won’t trade in his 1978 road hog for a 1981 version, and clunker buyouts in other states haven’t gotten them off the road; they’ve ended up being donated to worthy causes. Neither encourages the many drivers who use their cars more efficiently by combining trips or occasionally using public transportation.

Perhaps the most effective use of an incentive program is to combine the new-car/clunker idea and target new-car rebates to people who drive really foul vehicles. That would limit costs to all drivers and create more persuasive incentives to buy cleaner vehicles. To prevent people from simply buying old cars for the purpose of getting a rebate on a new one, lawmakers might require drivers to own the clunkers for at least a year and demonstrate regular use of it. After the rebate has been given, the clunker should be crushed.

Stationary sources of air pollution — energy plants, factories, incinerators, etc. — have been forced by state law to clean up their acts, at considerable expense. Vehicle owners have the same responsibility, but as with business, incentives should help move the process along.