Background:
Thailand´s policy on increasing graphic health warning areas on cigarette packs, from 50% to 55% to 85% has the objective of increasing knowledge and perception of harmful health effects of cigarette smoking. The objective of this field experiment study is to find out the effectiveness of the 85% graphic health warning label.

Methods:
Factorial design was implemented after the policy regulation was implemented in 2014. Recruitment of 724 students in 4 high schools and 5 colleges in Bangkok metropolitan area were chosen. Perceptions on intention to quit smoking were tested after introducing the students to 8 mock-up cigarette packs. The study was conducted in June of 2015 and 80 students per school were chosen.

Results:
Results indicated that 85% graphic health warning cigarette pack significantly increased fear arousal, and intention not to smoke, among non-smoking students. Among smoking students, particular type of picture and increasing size of graphic health warning interact significantly to increasing fear arousal and intention to quit.

Conclusions:
The impact of increasing size of graphic health warning on cigarette pack has significant influences on intention to quit among smoking- as well as non-smoking youths. This implied that plain packaging would be an ultimate strategy to increase awareness and perception of harmful health effects of cigarette smoking.