Indeed, even in San Francisco I measured indoor humidity as low as 18% before I got a humidifier, and dryer+colder places (e.g. Salt Lake City) could be even lo

Indeed, even in San Francisco I measured indoor humidity as low as 18% before I got a humidifier, and dryer+colder places (e.g. Salt Lake City) could be even lower inside. Humidity below 40-50% at indoor temperatures has noticeable negative effects for me. As other comments have mentioned, in the long run for maximum health and maximum energy efficiency we really need to design our buildings to be air tight, with energy-recovering ventilation systems and humidity control.