We need to 'alert' the public 'to' the dangers of these chemicals. [=we need to tell the public about the dangers of these chemicals]
Several neighbors 'alerted' the authorities/police when they noticed strangers acting suspiciously.
The nurse kept/stayed 'alert' for any change [=watched for any change] in the patient's condition.
(..) He used to go with her. - Which one is she? - From Maryland. She brought the chocolate-covered cherries you didn't like. Oh, yeah. Very alert. Warm person. Oh, yeah, she's great. So how come nothing materialized there? Well, it's a tough thing to talk about. (..)1990 TvShow: Seinfeld Title: The Stakeout Season: 1 Episode 2