One micron equals 1/1000 mm (1/25,000 in one inch). Airborne particles are typically measured in microns. The human eye can detect dust and debris about 25 microns in diameter. The size of the particle can be used to assess the level of danger to the health of humans.
The heat-pump water heaters (HPWH) cannot handle the latent load of clothes dryers vented to the interior. Each heat pump has a Sensible Heating Ratio (SHR), which tells you how much it can service the sensible and latent loads.
If there is no way to vent a clothes dryer to the outdoors, you need a ventless clothes dryer. You can choose between a ventless heat pump or condensing clothes dryer. Neither of these dryers must be vented outside.
Generally no. Unless the vent pipe goes into the garage and to the outside. Which would not be efficient piping.