I’ve stopped riding most public transportation.  

The reason I’ve stopped riding most public transportation in San Francisco is that the street people ride it often and their smell, in an enclosed space, can be unbearable to me.  I find many people have the same reaction as I.

Since I don’t expect the street people problem to be solved in the foreseeable future and I certainly don’t expect the smelly people on public transport problem to be resolved in the foreseeable future; I have a solution.

The law can ban unduly smelly people from entering public transport.  The problem with this solution is that leaving the decision of who smells too bad can not be left up to the vagaries of individual noses.

The solution is to use a meter at the entry point to buses and other transport.  The image of just such a meter is on the right.  This meter detects and measures ammonia. Ammonia is the main ingredient in the foul smell of all human beings.

I was unable to measure a sufficient large sample of foul-smelling people to get a reliable number to publish here.  

We have determined a reliable number for the alcohol breath test for drivers to measure drunkenness.  The same approach can be used for foul smelling street people.