To save production costs M class planets are described as being within human tolerances, suggesting the gravity pressure etc is close enough to not require life support. Any Near M Class planets visited have the actors wear the appropriate gear, be it breath masks or full body suits
Two words: Tri-ox compound. ProfessorTrek 02:42, July 18, 2011 (UTC)
Class L planets are also livable on but do take time to adjust to. However, to make it a lot simpler generally all the series' use class M planets so they don't have to show that, and also use Tri-ox compound.