When Bay Area skies turn hazy, your home should feel like a refuge, not a smoky extension of the outdoors. If you live in Atherton, you know wildfire smoke can drift in for days even when fires are far away. You want a simple, smart plan to keep indoor air cleaner, protect your family’s comfort, and maintain your home’s value.
This guide gives you a clear path: what filtration can and cannot do, which upgrades matter most, and how to size and place equipment in an Atherton home. You will also find a prioritized plan you can start before or after closing, plus a quick checklist and FAQs. Let’s dive in.
Why smoke days hit Atherton homes
Wildfire smoke is the main driver of poor air days in the Bay Area. In San Mateo County, multi-day smoke episodes can raise outdoor particle levels even when the fire is not nearby. That polluted air can infiltrate your home through small gaps, leaky ducts, and unfiltered ventilation.
Many Atherton properties are single-family homes with forced-air HVAC. That is good news because you can often upgrade your HVAC filter, run the fan to move more air through filtration, and add portable HEPA purifiers where you spend the most time.
Treat smoke readiness as both a comfort upgrade and a smart selling point. If you are selling, documenting recent filtration improvements builds buyer confidence. If you are buying, setting up a clean sleeping room and right-sized HEPA units helps you settle in with less disruption.
What filtration can and cannot do
Smoke is a mix of gases and fine particles. The health risk is dominated by PM2.5 and smaller particles. Filtration is very effective at reducing these particles indoors. It does not remove all smoke gases.
Your smoke-day goals are simple:
- Lower indoor PM2.5 as much as practical.
- Create at least one “clean room” where particle levels stay much lower.
- Avoid adding indoor particles from cooking or candles.
Know the limits. Particle filters do not remove carbon monoxide. If you suspect a combustion issue in the house, evacuate and address CO as a separate safety risk. Leaky homes or unfiltered outdoor intake will also reduce filtration gains until you seal and control those pathways.
Public health agencies recommend limiting exposure on high-AQI days, relying on HEPA filtration indoors, and using N95-type respirators when you must be outside.
The right filters and metrics
Choosing equipment is easier when you focus on a few proven options and simple math.
HEPA portable air cleaners
True HEPA captures at least 99.97% of particles at 0.3 microns. Portable HEPA purifiers are highly effective for wildfire smoke. Place them in rooms you use most, especially bedrooms, and run them on higher fan speeds during smoke events.
HVAC filters and MERV levels
The MERV scale rates how well in-duct filters capture particles. For wildfire smoke, upgrade to MERV 13 or higher if your system allows it. Many modern air handlers can handle MERV 13. Older or undersized systems may need a professional check to confirm fan capacity and static pressure.
During smoke events, set the HVAC fan to ON so more air passes through the filter. This improves whole-home particle reduction.
Carbon for odors and gases
Activated carbon or other sorbent stages reduce odors and some smoke-related gases. These inserts have limited capacity and can saturate faster during long smoke periods. Replace them as indicated by the manufacturer.
DIY clean air options
A Corsi-Rosenthal box (box fan plus panel filters) can deliver high clean air for low cost. Performance depends on build quality and materials. It is a strong budget option for living rooms or large bedrooms.
CADR and ACH sizing
Two metrics guide right-sizing:
- CADR (Clean Air Delivery Rate) is the cfm of clean air a unit delivers for particles. Use the smoke CADR.
- ACH (Air Changes per Hour) is how many times per hour the room air is filtered. Aim for multiple air changes per hour. A target around 4 to 6 ACH can significantly lower indoor PM2.5 in occupied rooms.
A quick formula:
- Room volume = length × width × height (ft). Example: 12 × 14 × 8 = 1,344 ft³.
- CADR needed (cfm) = (Room volume × ACH target) ÷ 60. Example: 1,344 × 5 ÷ 60 ≈ 112 cfm.
Pick a unit with smoke CADR at or above the number you calculate.
A prioritized upgrade plan
You do not need to renovate your whole system to feel a difference. Start with the highest-impact steps, then build from there.
High-impact first steps
Add portable HEPA in bedrooms and the main living room. Bedrooms deserve priority because you spend long, continuous hours there. Use the CADR math to size each unit and run on high during smoke days with windows and doors closed.
Create a clean sleeping room. Choose a bedroom with a good-fitting door. Add a right-sized HEPA unit, close supply vents if they leak, and seal obvious door gaps with weatherstripping. Keep the door closed during smoke peaks.
Upgrade the HVAC filter to MERV 13 if compatible. Run the fan continuously during smoke events to move more air through the filter. Check with an HVAC professional if you are unsure about compatibility.
Use carbon filters if odors linger. Select air cleaners with a substantial activated carbon stage when smell is a concern. Replace carbon filters promptly when performance fades.
Secondary improvements
Build a Corsi-Rosenthal box for large spaces. It is a cost-effective boost in living rooms or playrooms.
Seal air leaks and manage ventilation. Weatherstrip doors and windows. Add door sweeps and seal visible gaps. If your system has an outdoor air intake that is unfiltered, disable it during heavy smoke.
Inspect and seal ducts. Leaky ducts bypass your in-duct filter. A duct inspection and sealing upgrade improves filtration and comfort.
Consider higher-capacity whole-home filtration. Larger media cabinets (MERV 13–16) or in-duct HEPA systems can be added by a professional. These solutions may require a fan upgrade.
Room-by-room tips
- Bedrooms: Highest priority for HEPA. Keep doors closed during smoke days and use the unit on higher settings.
- Living room: Add a higher-CADR unit or a Corsi-Rosenthal box. Place it where air circulates freely without blocking intake or outlet.
- Kitchen: Avoid frying and broiling during smoke days. Venting can pull smoky air inside if it creates negative pressure. Only use the range hood if it is essential and outdoor air is acceptable.
- Garage: Keep the garage sealed from the house. Limit vehicle idling and traffic during heavy smoke.
- Mechanical area: Check filter rack size and fit. A well-fitted, thicker filter often performs better and lasts longer.
Pre-close and post-close moves
If you are selling, simple, documented steps can strengthen your listing story. If you are buying, move in with a plan that protects comfort right away.
Pre-close seller moves:
- Install a compatible high-efficiency HVAC filter and note the model and install date.
- Place at least one right-sized HEPA purifier in a key room.
- Document any duct inspection or sealing work.
- Weatherstrip doors or windows in priority rooms.
Post-close owner moves:
- Schedule an HVAC evaluation to confirm MERV 13 compatibility and consider a larger media cabinet or upgraded air cleaner.
- Add portable HEPA units for each bedroom and the main living area.
- Use carbon stages where odor control is important and replace as needed.
- Consider backup power for key rooms if you expect outages.
Checklist and maintenance
You will get the most from filtration when you combine sizing, placement, and steady upkeep.
Quick action checklist
- Check local air quality daily during fire season using trusted sources.
- Calculate room volumes and target ACH, then select HEPA units by smoke CADR.
- Upgrade to MERV 13 if your system allows it and run the fan continuously on smoke days.
- Create one clean room for sleeping and keep it closed during peaks.
- Reduce indoor particle sources by pausing high-heat cooking and candle use.
- Keep a simple record of filter types, install dates, and service.
Filter replacement basics
- Portable HEPA: Replace cartridges per the manufacturer. Expect faster loading during prolonged smoke events.
- HVAC filters: Check more often during and after smoke periods. A dirty filter reduces airflow and performance.
- Carbon filters: Odor control fades as carbon saturates. Replace promptly for best results.
Monitor your results
A consumer PM2.5 monitor helps you see if your home is performing well. Many purifiers include onboard sensors. Use them as a relative guide and compare indoor readings to outdoors. If indoor levels stay well below outdoors, your setup is working.
Estimated costs
- Portable HEPA purifiers: about 100 to 1,000 dollars depending on capacity and features. Many effective units fall between 200 and 500 dollars.
- Corsi-Rosenthal box: roughly 40 to 150 dollars in materials.
- MERV 13 HVAC filters: about 15 to 50 dollars each depending on size and thickness.
- HVAC inspection or duct sealing: from a few hundred to several thousand dollars depending on scope.
- Whole-home media cabinets or in-duct HEPA: often 1,000 to 5,000 dollars or more including labor based on the site.
Common pitfalls to avoid
- Using undersized units. If the CADR is too low for the room volume, you will not reach your ACH target.
- Running purifiers on low during smoke peaks. High settings make a big difference when outdoor AQI is poor.
- Ignoring air leaks. Even great filters struggle if unfiltered outdoor air is continually pulled inside.
- Letting filters sit too long. Overloaded filters reduce airflow and filtration.
When to call a professional
Check with an HVAC professional before making permanent filter upgrades or adding whole-home filtration. They can evaluate static pressure, fan capacity, filter cabinet size, and duct leakage so your system performs well on smoke days and every day.
If you are preparing to buy or sell in Atherton, smoke-day readiness can be part of a smarter home strategy. It improves comfort, supports health guidance, and gives you a simple value story to share during a sale.
Ready to align your home plans with Atherton’s reality on smoke days? Let’s talk about your next move and how to position your property with practical, high-impact upgrades. Connect with Rabeet Noor to plan your path with confidence.
FAQs
What MERV filter should I use for wildfire smoke in Atherton?
- Aim for MERV 13 or higher if your HVAC system is compatible. Have an HVAC professional confirm before a permanent change if you are unsure.
How many HEPA purifiers do I need for my home?
- Place one in each frequently used room, especially bedrooms and the main living area. Size each unit using smoke CADR to reach several ACH.
Do filters remove smoke odors and gases?
- HEPA and MERV filters remove particles like PM2.5. Odors and gases require activated carbon or similar media, which have limited capacity and need timely replacement.
Should I open windows briefly during a smoke event?
- Keep windows and doors closed while outdoor AQI is poor. Ventilate with outside air only when outdoor conditions improve or when you have filtered ventilation.
Will a MERV 13 filter stress my HVAC system?
- Many modern air handlers can handle MERV 13. Older or undersized systems may see reduced airflow. Have a professional verify fan capacity and static pressure.
What can I do if the power goes out during smoke days?
- Portable HEPA units need power. Consider backup options for a key room or relocate to a public space with filtered air if outages are prolonged.
