Noun and の adjective
えいせい
その歯科衛生士は、お口を開けてください、と私に言いながらにやりと笑い、日焼けした顔に白い歯をキラリと見せた。
As the dental hygienist told me to open my mouth I saw a flash of white teeth from her tanned face.
In defense of life, you have to remain clean. If you are dirty, you will die earlier (think: plagues). So, to defend said life, you must have good hygiene.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.