How much more Toxic is Ammonia than Urea?

Question

I learned in high school that the urea cycle turns the ammonia into urea because ammonia is significantly more toxic than urea. A quick internet search confirms this (1, 2). I started to wonder exactly how much more toxic ammonia is than urea, so I did some research. Hyperammonemia, the disease caused by the build up of ammonia in the blood, can fatal at 200 micromol/L in adults. I tried to find the corresponding value for urea, but ran into a problem.

Uremia is a disease caused by kidney failure and the build up of several toxins, only one of which is urea. However, since uremia is frequently measured using urea concentrations in the blood, internet searches for urea keep on circling back to uremia.

Does anyone know at what concentrations urea is fatal? Alternatively, is there another way to compare the toxicity of ammonia and urea?

Answer 1

Probably the easiest way to compare them is to look at their material safety data sheets (MSDS).

The MSDS for urea lists its LD50 as 8471 mg/kg, so it's not very toxic stuff. The US Occupational Safety and Health Administration (OSHA) does not consider it a hazardous material.

The MSDS for ammonia lists its LD50 as 350 mg/kg, so it's about 24 times more toxic than urea. OSHA definitely considers it a hazardous material. It has inhalation, contact, and ingestion dangers.

Note that the LD50 values were determined using rats, so they can only be taken as estimates in humans.