Of course it is harmful since you are obviously inhaling such toxic chemicals (yeah but we commute everyday and the pollution does the same...or the cigarettes, bleah ...im rambling) obviously ANYTHING and EVERYTHING we put in our bodies are potentially "harmful" (yes, the magnesium silicate in your Revlon lipstick WILL give you cancer! j/k...actually im not exactly kidding here because i read it somewhere but i think its a bunch of useless crap that gets recycled every now and then in all those womens magazines health articles) Anyway. If you are a generally unhealthy person, have difficulty breathing, are asthmatic, have allergies , et;al...well then inhaling a drug as blunt and toxic as methamphetamine will probably and MOST LIKELY do you more damage and make you feel worse than if you hadnt . Common sense should reign here, each individual is different and drug users MUST know their limitations and thresholds when it comes to their drug intake. I dont know anybody who died or had to be rushed to emergency because he kept the meth smoke in for three minutes and 32 seconds...but this may not apply to you because its MY reality and it doesnt mean im ignorant about how bad it is. I for one am very meticulous with my health because i want to enjoy my drugs to the fullest without being reckless ...i know thats a paradoxical statement but its the only way for me. P.S. IMO, and based on MY experiences, inhaling the smoke and exhaling it a little slower (like you make smoke rings , or let it glide outta your mouth kind of slow) DEFINITELY increases your hit.So IMO (again, i just have to repeat) , its not the length of time kept in your lungs...its how sslow you exhale it (try exhaling it through a straw ..youll get the picture.) hmm....i dont know if thats the same damn thing,(keeping it in, exhaling it slow) what the hell. 8) All i know is it doesnt hurt your throat(unlike tobacco, or weed, ugh), and exhaling it through your nose will guarantee a good hit. This is just my two cents worth and when in doubt, DONT.