It depends upon how close or intimate and comfortable you are with the person. If you are close or intimate and comfortable with them, find loving and caring words expressing concern placing it as important to them rather than a "selfish" request. If you do not have an endearing relationship, do not mention it at all, unless you are an adult using your efforts to teach a child. Or give them a spray as a present. Or when out and they smell bad spray yourself and then give them a spritz. If its a women find out if they have a period (it's a women's thing). If they don't, maybe they don't have any clean towels or clothes so they can have a shower and they are waiting for them to dry. Maybe they breathe in through their mouth instead of in through their nose, it makes a difference to the sense of smell. Do not tell them they smell bad straight up. I agree, you can't really go out and say it without being rude. If it's their breathe you could always take some gum and offer it for them also. I agree, it's a tricky subject. When my employees smell I tend to say general hints to the whole team such as "Let's remember proper hygeine, team. Especially in the summer months." That usually makes everyone on my staff rush off and spray or wash or brush teeth at their break. If you are not the boss, not a parent, not a relative, and not a friend to the offending person, you have to tread carfully. I often use this trick: I ask about myself in a confidential tone. Often I'll lean in and whisper "Can you help me? Do I smell bad?" and that makes the person take a whiff. Plus they trust you once you have confided your insecurity to them. The majority of the time, once they've told me I don't smell, they ask me to check their smell. I usually give a pretend whiff and say "well, only a bit." or "hey, it is hot after all!" And that is subtle enough to give them the hint that they need to fix their stench. Be really really subtle and polite in this situation. It's very embarrassing and offensive to tell someone about his/her body. Think of how you'd want to hear about it. Tact, deplomacy and delicacy are the way of dealing with bodily functions.