October 5, 2026
Got a Spare the Air Alert? Here's exactly what your HVAC system should be doing — and what to check before the next one hits.
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Your phone buzzes with a Spare the Air Alert. You close the windows, maybe turn on a fan, and figure you’ve done your part. But here’s the thing — if your HVAC system isn’t set up correctly, closing those windows might be the only useful thing you did. The air inside your home is still being circulated, filtered (or not), and potentially pulled in from outside through leaky ducts you’ve never thought about.
Santa Clara County’s air quality challenges are specific and serious, and your HVAC system is either working with you or quietly working against you. Here’s how to know which one it is.
Spare the Air is a program run by the Bay Area Air Quality Management District — the BAAQMD — that’s been in place since 1991. When air pollution is forecast to reach unhealthy levels, the BAAQMD issues an alert, and wood burning of any kind becomes prohibited throughout the entire Bay Area. That includes your fireplace, your fire pit, and your neighbor’s.
There are two distinct seasons for these alerts. Summer alerts, running roughly April through October, are triggered by ozone buildup on hot, sunny days. Winter alerts are driven by fine particulate matter — PM2.5 — from wood smoke and, increasingly, wildfire smoke that drifts in from fires burning in the foothills and beyond. In recent years, the two seasons have started to blur, with wildfire smoke showing up as early as July and lingering into November.
What makes Santa Clara County particularly vulnerable is its geography. The valley sits in a bowl, enclosed by the Diablo Range to the east and the Santa Cruz Mountains to the west. On still days, that topography traps pollutants close to the ground. Add easterly Diablo winds carrying smoke from Sacramento Valley agricultural burns, and you can have dangerous AQI levels even when no fire is burning nearby. The December 2024 Spare the Air Alert, for example, was triggered specifically by regional wood burning combined with easterly winds transporting pollution in from the Central Valley — a pattern we’ve seen play out many times over.
Yes — but how you run it matters more than whether you run it.
Most central AC and forced-air systems have two ventilation settings: one that pulls in fresh outdoor air, and one that recirculates the air already inside your home. During a Spare the Air event, you want recirculation mode. That way, your system keeps cooling and filtering the air that’s already inside rather than drawing in smoke-laden outdoor air with every cycle.
The problem is that a lot of homeowners don’t know which mode their system is in, and some systems default to pulling in outdoor air. If yours does that and you haven’t checked, running your AC during a smoke event could actually be making your indoor air worse — not better. It’s worth locating your thermostat settings or calling someone who can walk you through it before the next alert hits.
There’s a second issue that’s even less talked about: duct leakage. Even if your system is set to recirculate, leaky ductwork can pull outdoor air directly into the airstream before it ever reaches the filter. Ducts that run through attics, crawl spaces, or garages — which is common in the mid-century and 1970s-era homes throughout Willow Glen, Cambrian, and Almaden Valley — are especially prone to this. The seams dry out, the connections loosen, and gaps form that let unfiltered outside air bypass your system entirely.
Closing your windows during a smoke event is good advice. But if your ducts have gaps, you’re not actually sealed off from what’s outside. Duct sealing is one of the most effective steps you can take to protect your indoor air during wildfire season, and it’s one of the most consistently overlooked. A visual inspection won’t always catch it — a proper assessment involves checking for pressure differentials and airflow patterns that reveal where the leaks are actually occurring.
The BAAQMD offers free alerts through the Spare the Air app, available for both iOS and Android. You can also sign up for email and text notifications at sparetheair.org. The alerts go out the evening before a forecasted bad air day, so you have time to prepare rather than react.
Knowing the alert is coming gives you a window to act. That might mean checking your filter, switching your thermostat to recirculation mode, or making sure your portable HEPA purifier is running in the room where your family spends the most time.
If you’ve ever noticed that smoky smell creeping in despite closed windows and doors, that’s information worth paying attention to. Smoke is made up of extremely fine particles — PM2.5 — that are small enough to infiltrate through gaps in your building envelope, duct connections, and around electrical outlets. Homes with older construction, single-pane windows, or ductwork that’s never been inspected are particularly susceptible. In Santa Clara County, where a significant portion of the housing stock dates back to the 1950s through 1980s, this is a widespread and underappreciated problem.
The good news is that the fixes are real and they work. Sealing ducts, upgrading filters, and adding whole-home filtration equipment can make a measurable difference in what your family is actually breathing during a smoke event. But you can’t prioritize the right fix without knowing what’s actually happening in your specific system — which is exactly what a proper air quality assessment is designed to figure out.
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MERV stands for Minimum Efficiency Reporting Value. It’s the scale used to measure how effectively an air filter captures airborne particles, and it runs from 1 to 16 for residential and commercial applications. The higher the number, the smaller the particles the filter can trap.
Most homes in Santa Clara County are running filters somewhere in the MERV 4 to MERV 8 range — the standard fiberglass or basic pleated filters you find at the hardware store. Those filters are primarily designed to protect your HVAC equipment from dust buildup. They do very little for the fine particulate matter that makes wildfire smoke dangerous.
The EPA and ASHRAE both recommend MERV 13 as the target for residential HVAC systems dealing with wildfire smoke or poor outdoor air quality. The BAAQMD’s own wildfire safety guidance echoes this recommendation. A MERV 13 filter can capture particles as small as 0.3 to 1 micron — which covers the size range where wildfire smoke and PM2.5 live.
For most Santa Clara County homeowners dealing with a combination of seasonal allergies, wildfire smoke, and general Bay Area air quality, the MERV 11 to MERV 13 range is the practical sweet spot. MERV 11 handles pet dander, mold spores, and fine dust effectively. MERV 13 adds meaningful protection against smoke particles and the kind of fine particulate matter that drives Spare the Air Alerts.
If you have family members with asthma, allergies, or any respiratory condition, MERV 13 is worth prioritizing — especially given that the valley’s oak, cypress, and eucalyptus pollen season overlaps significantly with the early months of wildfire season. You’re often dealing with both at the same time, and a MERV 8 filter isn’t handling either one well.
Here’s where a lot of homeowners make a mistake: they assume that higher is always better, so they grab a MERV 14 or MERV 16 filter thinking they’re getting hospital-grade protection. The problem is that filters above MERV 13 are designed for commercial and medical environments with HVAC systems built to handle the increased airflow resistance. In a typical residential system, a MERV 14 or higher filter can restrict airflow enough to strain your blower motor, reduce heating and cooling efficiency, and potentially cause system damage over time.
The right filter is the highest MERV rating your specific system can safely support — and that’s not always obvious from the filter slot dimensions alone. It depends on your system’s static pressure tolerance, blower capacity, and duct configuration. During a Spare the Air event, the EPA also recommends changing your filter more frequently than usual. If you pull out your filter and it’s visibly dark or clogged, you’re already overdue — and your system has been working harder than it should while delivering worse filtration. During heavy smoke weeks, checking the filter every two to three weeks instead of the standard 90 days is a reasonable precaution.
Beyond the filter itself, there are other layers worth considering if smoke events are a recurring concern in your household. HEPA filtration systems installed directly into your HVAC airstream, UV lights that neutralize mold and bacteria circulating through the air handler, and whole-home air purifiers are all options that go beyond what a filter swap alone can accomplish. For homes with significant infiltration issues, an energy recovery ventilator can bring in fresh outdoor air while controlling the rate and filtering what comes in — solving the “sealed sick building” problem without sacrificing ventilation entirely.
An air quality assessment isn’t just someone checking your filter and telling you to upgrade it. A thorough assessment looks at your HVAC system as a whole — the filter, yes, but also the coils, the ductwork, the ventilation adequacy, and the moisture levels in your home. Humidity is a factor that often gets overlooked: the target range for healthy indoor air is 30 to 60 percent relative humidity. Below 30 percent, respiratory passages dry out and become more irritated by particulates. Above 60 percent, you’re creating conditions where mold can establish itself, which introduces a different category of air quality problem entirely.
A good assessment also looks for pollutant sources beyond smoke. Cleaning products, paints, adhesives, and solvents all off-gas volatile organic compounds — VOCs — that accumulate in tightly sealed homes. Combustion appliances like gas ranges and water heaters can introduce carbon monoxide and nitrogen dioxide if they’re not properly vented. These aren’t wildfire problems, but they’re indoor air quality problems that a smoke event can make worse by keeping windows closed for extended periods.
What you walk away with from a proper assessment is a prioritized list of what’s actually happening in your home, ranked by impact. Not a sales pitch for the most expensive equipment on the shelf — a real diagnosis. Maybe your ductwork is tight and your filter just needs an upgrade. Maybe there’s a ventilation issue in one part of the house that’s causing pollutants to accumulate. Maybe your system genuinely can’t support MERV 13 and a whole-home purifier is the better path.
We’ve been doing this work in San Jose and across Santa Clara County since 1985. That’s 40 years of Spare the Air seasons, wildfire events, and Bay Area air quality cycles. The valley’s bowl-shaped topography, the Diablo wind patterns, the aging ductwork in Berryessa and Los Gatos — we’ve seen how all of it plays out inside real homes. That context shapes how we approach an assessment, and it’s something no national chain can replicate by reading a zip code off a form.
The next Spare the Air Alert will come. It might be a wildfire pushing smoke across the Diablo Range, a cold still week in January trapping wood smoke over the valley, or a hot September afternoon building ozone over San Jose. When it does, the question is whether your home is genuinely ready or just feels like it is.
Upgrading to a MERV 13 filter, switching to recirculation mode, and signing up for alerts through the Spare the Air app are all things you can do today without any outside help. But if you’ve noticed smoke smell indoors, persistent dust, or allergy symptoms that don’t quit even when you’re home with the windows shut, those are signs worth investigating properly.
We offer free estimates, same-day availability, and 24/7 emergency service — including weekends and holidays. If you want to know what’s actually happening with your system before the next alert hits, reach out and we’ll take a look.
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