The Clean Air Act's requirements have sent emissions in the right direction. According to the EPA, since 1980 annual emissions of carbon monoxide are down 52%, emissions of ozone are down 41%, and emissions of nitrogen dioxide are down 37%. (Emissions of lead are down 97% thanks to taking the lead out of gasoline in the early 1980s).

The Obama plan could slow this progress. An economic phenomenon called "price elasticity of demand" is well established when it comes to automobile purchases. In other words, if you raise the price of new cars, people will buy fewer of them or, at a minimum, put off the purchase for a year or so while they drive the old clunker for a few thousand more miles. And fewer new cars means more pollution, which can cause significant health problems. Yet environmentalists and the press have ignored this issue, so as not to inject a note of complexity or doubt into the chorus of glee that greeted the president's attack on greenhouse-gas emissions. Ive been saying this the entire time

Last fall, however, both the press and the green community paid significant attention to a study conducted by researchers from California State University at Fullerton and Sonoma Technology Inc. The study showed that, from 2005-07, California's South Coast and San Joaquin Valley air basins experienced more than 100 days in which ozone levels exceeded the National Ambient Quality Standard. In each of those areas, more than 60% of the population was exposed to unhealthy levels of fine particulate matter.

The study concluded that if these areas had simply met the federal standard, these regions could have experienced 1,950 fewer new cases of adult-onset chronic bronchitis; 3,680 fewer premature deaths among those 30 and older; 141,370 fewer asthma attacks; almost 500,000 fewer lost days of work; and, importantly, avoided approximately $28 billion in total costs to the Southern California economy.

Clearly the health risks from fine particulates especially and also from smog are substantial. It is also true that many scientists and economists predict significant long-term costs associated with climate change. But the costs associated with excessive emissions of criteria air pollutants are immediate and observable.

The Obama fuel efficiency plan may also contribute to a significant increase in highway deaths as vehicles are required to quickly meet the new CAFE standard and will likely become lighter in weight as a result. According to a study completed in 2001 by the National Research Council (NRC), the last major increase in CAFE standards, mandated by the Energy Policy and Conservation Act of 1975, required about a 50% increase in fuel economy (to 27.5 mpg by model year 1985 from an average of 18 mpg in 1978). The NRC study concluded that the subsequent downsizing and down-weighting of vehicles, "while resulting in significant fuel savings, also resulted in a safety penalty." Specifically, the NRC estimated that in 1993 there were between 1,300 and 2,600 motor vehicle crash deaths that would not have occurred if cars were as heavy as they were in 1976.

The president now proposes a fuel economy increase of similar magnitude in an even quicker time frame -- to 39 mpg by model year 2016 from 27.5 mpg now. Given the time it takes for new technologies to be developed, tested and incorporated into new car models, it is likely that down-weighting of cars will be an important means of meeting the new standard. And one result again could be highway deaths that might otherwise not have occurred.

Well, one might argue, this would not be the case if everyone drove smaller cars. The NRC study considered this countervailing fact and included it in its estimates. But nearly half of all car crashes (more than 48% in the years studied) are one-vehicle crashes. Put another way: If your car hits a tree or a post or a bridge abutment, you are most certainly better off in a larger car.

If you want to imagine what things will be like here in seven years, you don't have to fantasize about a post-apocalyptic landscape of Tata Nanos. Just look at what Europe is like today.

Only things will be a lot better, because hybrid technology will have reached economies of scale.