Recent estimates from the Texas Commission on Environmental Quality (TCEQ) show that emissions of air pollutants that harm human health in the Houston region are falling dramatically. This excellent development is largely the result of very successful national regulations to reduce tailpipe emissions from motor vehicles. As the startling graph from the TCEQ below indicates, vehicle emissions in the Houston metropolitan area from have fallen by a half or more since 1999, and they are projected to fall further through 2030, despite a steady rise in vehicle miles traveled.

The pollutants in the graph – nitrogen oxides (NOx), carbon monoxide (CO), and volatile organic compounds (VOCs) – combine in the atmosphere to form ground-level ozone, which is very damaging to human health. Ground-level ozone leads to respiratory problems as well as "increased asthma attacks, increased hospital admissions, increased daily mortality, and other markers of morbidity."

Such an improvement is a result of increasingly strict federal emissions standards. The most recent “Tier 2” standards mandate dramatically lower emissions than vehicles a few decades ago. The graph below shows that new vehicles are mandated to emit at least 75% fewer pollutants than in the 1980s, and vehicles were even worse in previous decades. In some cases, pollution has fallen by 95% or more.

These emission standards rely on low-sulfur fuels. Sulfur content in gasoline has fallen from 300-350 parts per million (ppm) a decade ago to 30 ppm today.

Studies suggest that these standards have been cost-effective, with one exception from the Cato Institute. Before the Tier 2 regulations came into effect, the EPA estimated that total benefits would be 2.6-4.7 times greater than the costs,a figure that also included the rules for sulfur in fuels. More recently, a 2011 study from MIT found that marginal benefits of reducing the NOx from vehicles were 3.8 times greater than the average costs, and the standards should have been made stricter.*

Auto emissions standards are a regulatory success story. They have effectively improved air quality and health.



*The EPA estimated that benefits would be $13.9-$25.2 billion in 2030 (in 1997$), much higher than the expected cost of about $5.4 billion. Meanwhile, MIT estimated that the marginal abatement costs were $0.45/lb of NOx, while the estimated average benefit was $1.69/lb of NOx, and ideal emissions reductions for NOx in the automotive sector increased by approximately 15 percent.


The author would like to thank Dan Cohan, Assistant Professor of Civil and Environmental Engineering at Rice University, for providing guidance on this post.