Diesel exhaust fluid is the primary solution used for NOx aftertreatment. Today, most diesel vehicles cannot legally operate without DEF.

But what is NOx and why do fleets need DEF to treat it? Here’s a look at all the facts relevant to commercial vehicle fleets.

What is NOx?

NOx is a family of important air pollutants generated primarily by combustion. Diesel vehicles are a major source of NOx pollution.

In road transport, the term NOx represents two specific nitrogen oxides: nitric oxide (NO) and nitrogen dioxide (NO2). NOx is shorthand for all nitrogen oxides, but NO and NO2 are the relevant compounds among commercial vehicles. Regulators often only test for NO2 as an indicator of the broader group of oxides.

NOx is a pollutant because it can easily cause adverse effects on human and environmental health. Nitrogen oxides are volatile and react easily with other compounds, forming pollutants like ozone and fine particulate matter. Regulators have linked NOx to several human health problems. NOx is also a major contributor to smog and acid rain.

The EPA found that in 1999, vehicles generated about 50% of all anthropogenic NOx. Diesel vehicles are particularly susceptible to NOx generation: the high pressure, air content and combustion temperature of diesel create an ideal environment for NOx formation.

How are NOx regulated?

In the United States, two organizations limit NOx emissions from commercial vehicles through environmental regulations: the US EPA and the California Air Resources Board. EPA rules are federal regulations, while CARB rules only take effect in states that adopt the specific rules. Since the passage of the Clean Air Act of 1970, the EPA and CARB have steadily tightened vehicle NOx emissions with new rules every few years.

See also: Fleets Explained: Emissions Regulations