An evaporative cooler (informally "swamp cooler") drops dry hot outdoor air through a wet pad. Water evaporates, the air leaves cooler and much more humid, and the room is cooled at a fraction of the energy cost of refrigerant-cycle AC. They work spectacularly well in low-humidity climates (Arizona, New Mexico, interior California, the high desert, southern Spain, parts of Australia) and poorly almost everywhere else. DOE lists typical operating energy at one-quarter of equivalent refrigerant AC.
The humidity side is the trade-off. A unit cooling a house from 100°F to 80°F can raise indoor RH from 15% to 55-65% during operation. For a desert resident, the increased humidity is part of the comfort effect (skin evaporation moderates, dry-throat resolves). But the dashboard's humidity card will show a different shape than refrigerant-AC homes: RH rises with cooling instead of falling with it, and unless windows are cracked the indoor RH can drift into the dust-mite and mold range during sustained use.
The ventilation interaction is what most users get wrong. Evaporative coolers need exhaust pathway: a window or door cracked open on the opposite side of the house, sized so the cooler can move 20-40 air changes per hour through the structure. Without that exhaust, the indoor air saturates and the cooler stops cooling effectively, while RH climbs above 65% and condensation appears on the cool side of any wall cavity. The dashboard's indoor PM and CO2 will both stay low when this is set up right; both will misbehave when it is set up wrong.
Maintenance is the IAQ side. The wet pad and water reservoir are a microbial niche. Pads should be replaced annually, the reservoir bleached monthly during use, and the unit drained and dried at end of season. A neglected swamp cooler is a Legionella risk in older designs and a general mold/bacterial bioaerosol source in any design. EPA mold guidance applies. See reducing humidity for the off-season dehumidification piece.
References
- DOE - Evaporative coolers www.energy.gov
- ASHRAE - Technical resources www.ashrae.org
- EPA - Mold and health (humidity) www.epa.gov
- WHO - Indoor air quality: dampness and humidity www.who.int
