This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

adj. Extremely hazardous; excessively hazardous.

"In a letter to the Secret Service, Greenpeace and Friends of the Earth wrote that they were worried that security efforts focused on the Obama entourage "might not extend to the larger community which may suffer serious consequences in case of even one moderately successful terrorist release of ultrahazardous chemicals.""

ultrahazardous activity