NOx Isn’t Laughing Gas. Let’s Clear That Up.

Chemistry was never my strong suit in school. But I have picked up a little over the years covering diesel emissions regulations and engine oils.

If you’ve been in the industry very long at all, you’ve seen the term “NOx.” At first glance, it seems like just another one of those three-letter acronyms we love to throw around. DOT. HOS. ELD. CDL. SCR. DPF. DEF. ABS. OTR. (I feel like I could rewrite the “Initials” song from the musical “Hair.”)

But there’s a reason in journalism that the best practice is spelling out the full term before slinging around the acronym or abbreviation.

NOx vs. Nitrous Oxide: Yes, There’s a Difference

In writing about the latest efforts of the Trump administration to address the diesel exhaust fluid derates that have frustrated the industry for years, I noticed something that made me do a double-take: The Environmental Protection Agency referred to NOx as nitrous oxide.

Hold on… nitrous oxide is the stuff dentists use to relax you before they start drilling. It’s also known as laughing gas and sometimes used by young people as a recreational drug. Hippy crack.

But that’s not the emissions being regulated coming from diesel engines. What we’re actually talking about here is nitrogen oxides.

Why This Mix-Up Isn’t Just Semantics

Ah, yes, those pesky chemical abbreviations! As I said, they’re not something I know off the top of my head. But it does seem like we should be able to expect better from the EPA.

So, for the press release and fact sheet writers at the EPA, and for those of you who, like me, aren’t exactly chemistry experts, here’s a quick look at the difference between nitrous oxide and nitrogen oxides.

(EPA did get the distinction right in this DEF overview: https://www.epa.gov/regulations-emissions-vehicles-and-engines/diesel-exhaust-fluid)

Nitrous Oxide: Yes, the Laughing Gas

Nitrous oxide’s chemical formula is N₂O.

In addition to being used by dentists and young people wanting to get high, it’s also something that has been on the EPA’s radar, as it is a greenhouse gas. In fact, it’s considered the third most important to climate change after carbon dioxide and methane.

But N₂O comes largely from agriculture, from the use (some say over-use) of nitrogen fertilizers.

While some nitrous oxide does come from fossil fuel combustion, it’s not the byproduct that was addressed in diesel engine emissions regulations.

What Are Nitrogen Oxides and Why Are They Regulated?

Nitrogen oxides, or oxides of nitrogen, is a name for a group of emissions that includes nitrogen dioxide (NO2), which damage the human respiratory system and contribute to smog and acid rain.

These compounds form during high-temperature combustion, when nitrogen and oxygen in the air react inside the engine.

In fact, transportation is responsible for more than half of NOx emissions. That’s why modern diesel engines rely on systems such as selective catalytic reduction (SCR) and diesel exhaust fluid (DEF) to reduce NOx emissions.

When the EPA published its most recent NOx emissions rules in 2022, it projected that by 2045, it would reduce NOx emissions from the in-use fleet of heavy-duty trucks by almost 50%.