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There is More to School Bus Emissions Than NOx

While many school bus manufacturers tout the cleanliness of their school buses based on NOx emissions, there is more to emissions than just NOx. Learn more about the core emissions designated by the EPA and how school bus fuels stack up when all emissions are considered.


There is much debate about the cleanliness of school buses today, particularly surrounding diesel and propane engines. Both are touted as clean burning and emissions data being thrown around in the industry can make you dizzy and, quite frankly, confused. The truth is, based on EPA-regulated emissions, clean-diesel school buses today are comparable to or cleaner than other fuel types like propane, CNG and gasoline.

“Propane school buses are being lauded as the cleanest in the industry,” said Caley Edgerly, president and CEO of Thomas Built Buses. “We produce propane school buses, so of course we would stand behind that statement if it were true. Unfortunately, we can’t unequivocally say that propane is the cleanest fuel for school buses today. Based on EPA certifications, propane provides the lowest level of NOx in the industry. This is true.  But when considering the other EPA-regulated emissions like particulate matter, carbon monoxide and non-methane hydrocarbon, propane is not the clear winner in all of the four critical emissions categories. All engine types have nearly the same emissions footprint, and this footprint has improved dramatically compared to just ten years ago.”

NOx is a Small Part of a Larger Story

In 1970, air quality became a hot issue in the United States, which resulted in the enactment of the Clean Air Act. Overseen by the EPA, air quality nationally is continually monitored and compared to EPA federal emissions standards for six primary pollutants: carbon monoxide, nitrogen dioxide, sulfur dioxide, particulate matter, hydrocarbons and photochemical oxidants. These pollutants are considered the worst of the worst based on their effect on both human health and the environment.

School buses contribute to four of these six pollutants: carbon monoxide (CO), nitrogen dioxide (NOx), particulate matter (PM) and hydrocarbons (NMHC).  Over the years, EPA standards for these four emissions have become more stringent, and will likely continue to tighten in the future. Which is why all of these emissions are driving the future of school buses and engine manufacturing.

Today, all new clean-diesel, compressed natural gas, propane and gasoline-fueled school buses not only meet but exceed 2017 EPA federal emissions standards.  Take a look for yourself.


As you can see, some engines are lower in some emissions and higher in others. But across the board, all are clean, based on EPA emission standards.

Diesel Emissions 2019

Why Can’t I Just Rely on NOx Levels?

There’s a reason the EPA regulates several emissions for the school bus industry and not just NOx. Each pollutant has its own unique but critical impact on the environment and human health. Therefore, all of these core pollutants must continue to be monitored and regulated. Here’s a quick snapshot of why each pollutant matters.

  • Particulate Matter (PM) – Particulate matter includes tiny solid and liquid particles that are suspended in the air we breathe.  While some of these particles are large enough to be seen, other particles are so small they can only be seen with an electron microscope. In fact, the smaller the particles, the more hazardous they are to your health because they can embed deep within your lungs and sometimes can get into your bloodstream. Particulate matter can cause serious health issues including breathing problems, lung tissue damage and even cancer.  Across the board, Thomas Built buses have a near-zero PM rating.  
  • Nitrogen Oxide (NOx) – Nitrogen oxides are highly reactive gases most commonly known for causing acid rain.  What you may not realize is that when NOx reacts with ammonia, moisture and other compounds in the air, it creates microscopic particles that can cause emphysema or bronchitis and may aggravate heart disease and asthma.  New clean-diesel engines, like the Detroit™ DD5™, have extremely low PM levels and a NOx rating half the EPA standard.
  • Carbon Monoxide (CO) – Carbon monoxide is a colorless and odorless gas that also is a greenhouse gas. While not dangerous in lesser amounts, elevated levels can be absorbed into your red blood cells in place of oxygen, which may be lethal. While the standard is 15.5 grams per brake horsepower per hour, the upcoming DD5 diesel engine’s CO rating is 0.1 (less than 1% of the limit).
  • Nonmethane Hydrocarbons (NMHC) – Nonmethane hydrocarbons cause ground level 03 (OZONE) and smog. Breathing ozone can cause chest pain, coughing, throat irritation and airway inflammation, as well worsen bronchitis, emphysema and asthma. With a new and improved aftertreatment system, new clean-diesel engines will run at about 10% of the limit.


“With more than 480,000 school buses on the road every day, the impact of emissions can be great, which is why manufacturing safe and clean school buses is our number one priority,” said Edgerly. “All Thomas Built school buses including propane, CNG and clean diesel, not only meet EPA and GHG standards, but exceed them. This is true today and will continue to be the case well into the future.”


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