Am I sinful for involuntarily inhaling and being affected by drug smoke while on a bus with drug users during a scientific conference essential for my studies?

You committed a major sin by attending a conference with drug users for several reasons, including: 1. Your voluntary presence at a forbidden act without necessity makes you like the one committing it, as Allah Almighty says: "And He has already revealed to you in the Book that when you hear the verses of Allah being denied and ridiculed, do not sit with them until they enter into another conversation. Indeed, in that case, you would be like them." (An-Nisa: 140). 2. Your exposure to drug smoke poses a significant harm to you, and the Prophet, peace and blessings be upon him, prohibited harming oneself, saying: "There shall be no harm or reciprocating harm."

Based on this, attending the conference was not permissible for you, even if it was for study, because it leads to what is unlawful. An exception to this is if you were unaware of the presence of drug users and found no means to avoid them after becoming aware; in that case, you would be excused.