Health officials all recommend that central vacuum systems should exhaust outdoors while most installers do not put in an outdoor exhaust pipe.
A few conflicting facts:
The cost of piping the exhaust air outdoors is about $75 installed.
Central vacs are noisy and a few municipalities have begun to outlaw outdoor exhausts, primarily for the noise. Some people remove their filters and then an outdoor exhaust can even become a neighbourhood dust problem.
Manufacturers are making better and better filters to trap the dust, saying that they don't need to exhaust outdoors because they catch it all. In fact they are mostly responding to price pressure, as most of them will offer the outdoor exhaust as an option. Few consumers take the option.
Health officials say that the only harmful particles in the exhaust stream are those that are too small to be trapped by all those filters -- you would need a HEPA filter, and that just makes little sense on a central vac. HEPA filters are expensive and they seriously reduce the vacuum pressure, although we are seeing some HEPA filters on portable machines. Follow this link for more information on Indoor Air Quality.
When I see the health reports about what can run through a vacuum cleaner I want to get the stuff out of the house. When I see carpet studies that show that good regular vacuuming can actually use the carpet as a trap and have an even healthier house, as long as that 'trap' is cleaned out regularly, I like central vacs with outdoor exhausts even more. Then when I listen to my own vac blasting away next to the patio I wonder why science has never been applied to this appliance. If we can take a motorcycle and make it silent, don't tell me we can't do that with a vacuum cleaner. I have a muffler on mine, a little foam tube, but it doesn't do much.
I definitely recommend always exhausting a central vac outdoors, but I am going to begin researching how to make them quieter. Come back as this will be an ongoing database entry.