life-threatening vs serious

life-threatening and serious both are adjectives.

Word	Noun	Adjective	Verb	Adverb
life-threatening	No	Yes	No	No
serious	No	Yes	No	No

As adjectives, life-threatening and serious are synonyms defined as:

life-threatening and serious: causing fear or anxiety by threatening great harm

Other synonyms of life-threatening include dangerous, grave, grievous, severe.

life-threatening (adjective)	serious (adjective)
causing fear or anxiety by threatening great harm	concerned with work or important matters rather than play or trivialities
	completely lacking in playfulness
	causing fear or anxiety by threatening great harm
	requiring effort or concentration; complex and not easy to answer or solve
	of great consequence
	appealing to the mind