Relative humidity is the percent of moisture the air is currently holding compared to the maximum it could hold at that temperature. Healthy indoor air sits between 30% and 50% RH. That window is not arbitrary; it sits at the dip between two opposing health curves, made famous by the 1986 Sterling et al. paper.
Below 30%: mucous membranes dry out, skin cracks, static electricity climbs, wood furniture splits, and, well-documented, airborne viruses (influenza, SARS-CoV-2) last meaningfully longer in the air. Hospitals are increasingly required to stay above 40% for this reason. Above 60%: mold growth accelerates on cool surfaces, dust mites multiply (they need 60%+ to reproduce), and the air starts feeling muggy.
Winter in a heated home tends to run dry; summer in a humid climate tends to run wet. A simple plug-in humidifier or dehumidifier closes the gap. If RH is high and CO2 is high, the cause is people in a closed room and ventilation fixes both. If RH is high and CO2 is low, you have a moisture source (bathroom, kitchen, basement leak) and need to track it down.
The sensor reads RH from the Sensirion SEN66's integrated humidity channel, factory-calibrated to ±2% over the 20–80% range.
References
- ASHRAE Standard 55 - Thermal Environmental Conditions www.ashrae.org
- EPA - Mold course: water & moisture www.epa.gov
- Sterling et al. (1986) - Humidity comfort zones www.aivc.org
- CDC / NIOSH - Indoor environment & temperature www.cdc.gov
