Why Indoor Air Quality Drops in Austin's Cooler Months

Austin winters are mild enough that people assume poor winter indoor air is a northern problem — something that happens in houses sealed against snow. The mildness is exactly what causes it here.

Between late November and February, a Central Texas house is shut for a different reason than it is shut in July, but it is just as shut. And several things line up in the same ten or twelve weeks.

Four things happening at once

The house stays closed regardless. At 45°F nobody opens a window. At 68°F on a December afternoon nobody thinks to. Either way the air inside is not being exchanged with the air outside except through gaps and whatever the HVAC does.

Cedar peaks in the same weeks. Ashe juniper releases from roughly mid-December through February. It comes in on coats, on shoes, on dogs, and through the gaps. Then it settles into fabric — upholstery, bedding, rugs, curtains — and gets lifted back into the air every time somebody sits down. That is a genuinely seasonal indoor load and it is the largest one of the year here.

The first cold snap burns off the dust. A furnace or a set of heat strips that has been idle since March runs for the first time and burns off months of settled dust on the heat exchanger. That smell in the second week of November is real, and it comes with particles.

The system runs far less. This is the counterintuitive one. In July the AC runs twelve hours a day, and every one of those hours pushes the whole volume of the house through a filter. In a mild Austin winter the system cycles rarely. Air sits. Whatever is in it stays in it, and the filter you are relying on barely sees the air at all.

Add more time indoors, more cooking, more candles and more people in the house over the holidays, and the load goes up while the mechanism that removes it goes quiet.

What helps, in order

1. The filter, first. People associate filters with summer and skip them in winter, which is precisely backwards for a season where the filter gets less air across it and the indoor load is higher. Fresh filter in late November.

2. Run the fan. Most thermostats have a fan setting separate from heat and cool. Setting it to run continuously, or on a circulation schedule, pushes air through the filter even when the system is not calling for heat. This is the single change that does the most, and it costs a few dollars a month.

3. Damp-dust, never dry. A feather duster on a cedar-season dust load is an aerosol dispenser. A damp microfibre cloth captures. Free, immediate, and the difference is visible within a week.

4. Bedding weekly, hot. Bedding is the largest fabric surface in most houses and it is the one your face is in for a third of the day. Dry it indoors during peak weeks rather than on a line, where it collects on the way back in.

5. HEPA vacuum on soft surfaces, twice a week. A vacuum without proper filtration exhausts fine particles back into the room. If you change one piece of equipment for the winter, change that one.

6. Entry control. Shoes off, a mat that gets washed, and a wipe-down for the dog after a walk in January. Most of what is in the house walked in.

7. One deep pass in mid-January. Upholstery with the cushions out, curtains, under and behind the furniture. This is the one that resets the reservoir rather than skimming it, and it is the one that gets skipped.

What a cleaning crew cannot tell you

We do not test air. Nobody wiping baseboards can tell you what is in your air, what your particulate count is, whether there is a combustion issue with your heating, or whether what somebody in the house is feeling comes from cedar or from something else entirely. Those are questions for an indoor air quality assessment, an HVAC technician, or a doctor, depending on which one it is.

We also do not clean ducts, and cleaning does not change the pollen count outside your door. On a heavy release day, an open window undoes a great deal of work in ten minutes.

What is genuinely in scope is the settled reservoir: the dust and pollen already sitting on your surfaces, in your rugs and inside your upholstery, waiting to be put back into the air the next time somebody crosses the room. That reservoir is real, it is measurable in how a house feels, and it is the part of the problem that responds to cleaning.

The November order of operations

New filter, fan set to circulate, bedding washed, one deep pass on soft surfaces before the holidays. Four things, in that order. Then hold the weekly rhythm through February and reset once more in mid-January, which is the deepest point of cedar season and the point at which the reservoir is fullest.