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We’ve all seen it, we’ve all smelled it. Diesel exhaust. That ominous black cloud billowing from the exhaust pipe of the truck in front of you. You know the stuff is bad — bad for your health and bad for the environment — the way burns your throat and leaves a sooty stain in the air.

So what exactly is diesel exhaust, and why is it so bad?

Diesel exhaust is simply the byproduct of the combustion of diesel fuel. The black stuff spewing from the tailpipe is composed of a variety of potentially harmful pollutants. The components of diesel exhaust come in two forms: gases and particulate matter (commonly called soot).

The gaseous ingredients of diesel exhaust include carbon dioxide, carbon monoxide, sulfur dioxide, nitrogen oxides (also known as NOx gases), hydrocarbons, and polycyclic aromatic hydrocarbons (PAHs). None of these gases are things that we want in the air or in our lungs.

Carbon dioxide is the most infamous greenhouse gas. Carbon monoxide is notorious for being fatal to humans in high concentrations. Sulfur dioxide is the main culprit behind acid rain and can also cause respiratory problems when inhaled. Nitrogen oxides are toxic when inhaled, give smog its brown coloring, and contribute to ground-level ozone pollution. Hydrocarbons also contribute to ground-level ozone and can be carcinogenic. PAHs are potent air pollutants that can cause a variety of health problems, from birth defects to cancer.

The particulate phases of diesel exhaust are a serious health concern. Fine and ultra-fine particles of organic compounds, elemental carbon, and toxic heavy metals can cause serious respiratory problems as well as mutate cell DNA (which spurs the onset of cancer). The smaller the particle, the farther it can potentially be deposited into the lungs. Some diesel particulates are so small that they can reach the alveoli — the sacs in our lungs responsible for delivering oxygen to our blood.

People that live in urban neighborhoods with heavy vehicular traffic, especially in the form of trucks and buses, tend to suffer from disproportionately high rates of respiratory problems like asthma. The reason? Many factors can contribute to the onset of health problems like asthma, but in these cases, exposure to diesel exhaust is certainly a major contributor. Read more here.

Diesel exhaust is nasty stuff. So why even use diesel fuel?


Diesel fuel is appealing because it is significantly more efficient than regular gasoline. This means fewer stops at the filling station, which is ideal for shipping trucks, public transportation bus fleets, and school buses.

Luckily, diesel fuel has become much cleaner in recent years. As of 2010, all diesel fuel sold in the U.S. is ultra-low-sulfur diesel (ULSD).  In compliance with strict EPA mandates, this diesel contains a maximum of 15 ppm of sulfur. Removing sulfur from the fuel drastically decreases tailpipe emissions of sulfur dioxides, NOx gases, hydrocarbons, and other hazardous diesel particulates — by over 95 percent.

Improvements to diesel engines have also contributed to a reduction of pollutants in diesel exhaust. Many diesel vehicles have been retrofitted with a diesel particulate filter, which can prevent up to 85 percent of diesel soot from reaching the air. Most modern diesel cars come with the filter.

An additive called urea is now widely used as a “cleanser” for diesel vehicles.  Urea is an organic compound that reacts with NOx gases, removing oxygen molecules and transforming the harmful pollutant into simple nitrogen. The urea is injected into the exhaust stream before it exits the vehicle. When used in combination with a diesel particulate filter, urea tank systems can nearly eradicate tailpipe NOx emissions. By law, all diesel trucks in the U.S. must be outfitted with a urea tank.

Transitioning to widespread measures to reduce dangerous diesel exhaust emissions has its costs. ULSD is more expensive than regular diesel and more tedious to produce. Diesel particulate filters can be difficult to maintain; they must be monitored carefully and cleaned frequently to avoid clogging and overheating. Urea tanks must be refilled frequently and the urea itself is an additional expense.

But the environmental and human health benefits of cleaning up diesel fuel are obvious. Is it worth the extra costs? What else can we do?  Let us know…