Aerosol and gaseous emissions are generated in toilets. Drawing exhaust air from the ceiling ensures effective dissemination of airborne contaminants. Why is the exhaust air not drawn from the toilet instead?
- it would complicate toilet designs and make toilet bowls more expensive to manufacture
- it would require an electrician to install, as well as a plumber, greatly increasing the cost of installation
- the exhaust vent would suck in splashed urine and faeces-laden bowl water, turning the outlet pipe and the exhaust fan at the end into a stinking health hazard
- sitting on a vented bowl in winter would result in cold air rushing past your genitals
- you are conflating emissions and contaminants — bad (but otherwise harmless) smells rise up from toilet bowls, but the nasty bugs are only transmitted by splashing or human activity, so a ceiling exhaust does not help spread contaminants in any meaningful way because it does not generate strong air currents near the bowl
- exhaust vents in toilets have nothing to do with sanitation — they exist merely to maintain air quality (i.e. prevent the room from smelling bad)
- given that most replacement air will enter the room underneath the door, a vented toilet bowl will simply draw air along the floor — this will make the extraction of warm, smelly air that has risen towards the ceiling difficult, allowing smells to linger and requiring the exhaust fan to run significantly longer to accomplish its task of clearing the air (with a corresponding increase in electricity consumption and failure rate)
- if the toilet becomes blocked for whatever reason, the water level in the bowl could rise to the level of the vent and then get sucked directly into the exhaust pipe and down that pipe towards the electric fan at the other end — a scenario that won't end well regardless of what happens next
Having said all of the above, a number of waterless/composting toilets are vented and do work well — so the idea does have merit in certain circumstances.
Because the design of the bowl (assuming a "standard" toilet with flush) is such that it forms an airlock (the U bend) to stop the smell from the drain entering the bathroom.
They do sometimes. Look up long drop toilet. They're used sometimes at Western campsites. The vent is usually painted black. It heats up with solar radiation and causes a strong convection current that really sucks on the cheeks. They're surprising ly effective.