Final answer:
Peer pressure is a critical environmental factor that can trigger the onset and relapse of substance-related disorders.
Step-by-step explanation:
The critical environmental factor that can trigger the onset and relapse of substance-related disorders is peer pressure. Peer pressure refers to the influence one's friends or peers can have on their behavior, particularly when it comes to substance use or abuse.
For example, if a teenager is surrounded by friends who regularly engage in substance use, they may feel pressured to fit in and try drugs or alcohol.
This environmental factor can be a significant risk factor for developing substance-related disorders, as individuals may feel compelled to continue using substances to maintain social connections and avoid feeling left out.