For urea I found this page:
Unfortunately, the spraying is now becoming as we gather evidence as bad or worse than anything we could have imagined. The only situation I can remember worse than this was 20 or more years ago when populated areas were sprayed with Malathion and many people got sick, got cancer and some died. Since there doesn't seem to be anyone listening to the people on Monterey Peninsula and since no one seems to care a whit about our wishes in this I'm beginning to get scared of what I'm seeing. It is starting to take on a macabre quality and I think the rest of California and the rest of the states need to be prepared for this experience where you live eventually too. Because nothing is being done to stop the Australian light brown apple moth from moving county to county and state to state on ornamental plants, trees, and crops in the form of 300 to 1500 eggs per female moth. Though there are some laws to prevent some crops out of state there is no legislation yet that I know of to prevent the spreading of ornamental plants and trees by both individuals and companies. The eggs of the moth are so small that almost no one would find them until it is to late for your area. Since this moth has likely been in Northern California for several years before it was discovered this spring it is likely already in pockets in the neighboring states like Oregon, Nevada and possibly Arizona and Baja California. It is very likely in Arizona and Baja because it is already in Los Angeles County as it is one of the 11 counties in California that the moth has been found in.
Hazards of Urea according to Wikipedia:
Urea can be irritating to skin and eyes. Too high concentrations in the blood can cause damage to organs of the body. Low concentrations of urea such as in urine are not dangerous.
Repeated or prolonged contact with urea in fertiliser form on the skin may cause dermatitis. The substance also irritates the eyes, the skin and the respiratory tract. The substance decomposes on heating above melting point producing toxic gases. Reacts violently with strong oxidants, nitrites, inorganic chlorides, chlorites and perchlorates causing fire and explosion hazard.