Odds of lung cancer in 28 year old male. never smoked regularly, used to smoke occasionally socially <100 total cigarettes? 4+ years with no smoking.

Very rare: Lung cancer does occur but it is extremely rare in your age group(Under 30 years old). So I would not think about it or worry about it at all. But eat healthy(mainly plant based foods) and exercise(Brisk daily wals for 30 minutes is best) daily. This is the recipe for long term good health!!

Congrats: Although even a single cigarette can impact the lung tissue, you are not at higher risk to develop lung cancer with just 100 cigarettes. The trick is to avoid the temptation given how addicting cigarettes can be.

Highly unlikely: Lung cancer mainly occurs in older people. About 2 out of 3 people diagnosed with lung cancer are 65 or older, while less than 2% are younger than 45. The average age at the time of diagnosis is about 70. These facts plus having smoked only 5 packs of cigarettes total would make it highly improbable that you have lung cancer.