This evening I had a lesson with "little old man" (LOM) in my 4.5 tatami mat living room (incidentally, the Wikipedia link shows the exact layout of my room in its first sample diagram which shows a square). In the past, I've had lessons with the door between the bedroom and living room open because the bedroom was the only one with an air conditioner and I'd have to leave it open in the summer heat. Since the living room now has its own air conditioner, it's most efficient to close up the living room and cool only one small room.
This definitely makes for less wasteful energy consumption but it also puts me in a smallish room without much air movement because I use the A/C on a low power, "weak" mode to keep from freezing my students to death since most of them are more sensitive to cold than heat. Unfortunately, it also pens me up in a little room with all the attendant odors of the other party. Most Japanese people are pretty clean though sometimes the women go overboard on the perfume and that can be an issue. The bigger problem though, is halitosis.
Two of my students, including LOM, frequently have halitosis and even though I sit at least 4 feet away from my students when they speak, I am constantly bombarded by this unpleasant odor. This evening, after LOM left, I returned to the living room and found that the entire room now smelled of his bad breath. I had to open it up to the other rooms and crank up the air conditioner power to dissipate the smell. I guess that such a small room will really fill up with a smell when one person is doing a lot of talking.
I'm not the first person who lives in Japan who has remarked on the bad breath of Japanese people (which does appear to be a more frequent issue than it is back home). People often speculate that they aren't engaging in adequate oral hygiene but I honestly don't believe that is the case. In fact, despite the plethora of people running around with intricate metal dental work in Tokyo, I think Japanese people are pretty meticulous about brushing their teeth and many of them brush more often than Americans.
One of my dentists told me that one of the reasons Japanese people have more teeth problems which results in more fillings and bridgework is that the water isn't fluoridated nor is much of the toothpaste. Among older people, I think there may be a greater tendency toward bad teeth because dairy products weren't consumed with great regularity in the past and people didn't get enough calcium. This is the same reason you see older ladies who are bent over from osteoporosis. Incidentally, the reason you sometimes see more metal dental work in their mouths compared to westerners is related to the type of dental work national health insurance covers. I believe porcelain crowns aren't covered under the insurance plan so you don't see many transparent fillings or bridges.
So, I don't think that Japanese people aren't brushing. I can't say exactly why there are people who seem to have fowl breath in greater abundance here but it may have something to do with a diet rich in certain types of protein (fish, tofu, soy-based foods). Protein trapped on the tongue is a common cause of bad breath and toothbrushing doesn't necessarily cleanse the tongue. It could also have something to do with the types of toothpaste sold in Japan though I can't speak from personal experience since I've never used Japanese toothpaste (we always get imports). And though I'm sure this is a somewhat controversial bit of speculation from a medical viewpoint, it could be that they just eat less often and eat less food and people with empty stomachs seem more likely to have bad breath. One of my skinny (and favorite) college professors sometimes asked me if she had bad breath because she didn't eat much and knew it could be a problem.
None of this understanding really makes my life any easier though. It's still a bit of a struggle to get through an hour with someone who is exhaling foul air in my direction, even though I know he can't help it. I find myself trying to move back further and suppressing a cringe every time LOM leans forward bringing him a good foot and a half closer to me. I guess this is just one more reason to be glad when summer is over and I can open up all the rooms and windows again. It'll be "a breath of fresh air" in more ways than one.