Is DEF Toxic?

By now, you should know what SCR and DEF stand for, but if you don't I'll recap: SCR stands for Selective Catalytic Reduction, the technology that most of the major diesel engine manufacturers have chosen to meet the EPA's 2010 diesel engine emissions standards, and DEF stands for Diesel Exhaust Fluid, the urea solution that is injected into the aftertreatment system of a truck with an SCR engine.

Last month at the Mid-America Trucking Show in Louisville, KY, you could hardly go five minutes without someone talking about SCR and DEF, and the conflict that has erupted between four OEs that will be using SCR and DEF in 2010 (Daimler, Cummins, Volvo and Mack) and one that will be using "Advanced EGR" (Navistar/International).

It's no secret that Navistar executives have been very publicly criticizing the SCR/DEF technology package, making claims that SCR systems lay the burden of complying with EPA 2010 emissions regulations squarely on the shoulders of the driver, and suggesting that DEF becomes a "volatile" substance under extreme heat. Beyond the technical complaints, they have also suggested that DEF may not be as readily available to drivers out on the road as the SCR folks claim, a scary thought indeed.

The truth is that many of the claims and counter-claims made by the manufacturers can't be validated until 2010 trucks and engines are on the road a year from now. But, as I sat through nearly 20 hours of "EPA 2010" presentations in Louisville last month, it occurred to me that there is one issue that we can lay to rest right here and right now. Is DEF a "volatile" substance, or is it, as the SCR engine folks claim, about as hazardous as Windex?

Of course we all know there's one way to find out if a substance is dangerous: you check its Material Safety Data Sheet (MSDS). You already have a binder in your shop filled with MSDSs (or you should) rating the hazards of every chemical in your shop.

I went online to the website of Terra Industries, a DEF supplier that was represented at the "SCR OEM Summit," and I looked up the MSDS for DEF. You can see it for yourself at

Turns out, both sides can make their points: yes, DEF can emit harnful ammonia vapors when and if it degrades, but--realistically--the biggest risk posed by a DEF spill in your shop is that the floor will be slippery.

If you are considering purchasing new trucks with 2010 engines, it's worth your while to read the MSDS for DEF. Like all chemicals, DEF should be handled with care, but the MSDS makes it quite clear that DEF is NOT a hazardous substance.

Score one for SCR.