How to Keep the Air Inside Your Home Clean During Wildfires

How to Keep the Air Inside Your Home Clean During Wildfires

**Wildfire smoke is a growing reality for millions of Americans. Here's the science-backed playbook to protect the air you breathe inside your home—from simple AC tweaks to affordable DIY filters that work as well as commercial purifiers.**

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## Introduction: The Invisible Threat in Your Home

The eerie glow of an orange sun through thick wildfire smoke might be unnerving, but the biggest risk from the haze is something much harder to see. Tiny particles in the air—fine particulate matter known as PM2.5—can cause a range of short-term and chronic health issues.

Wildfire smoke can settle over cities for days, creating hazardous air for millions of people across multiple states. The immediate health effects are well-documented: coughing, shortness of breath, eye irritation, and worsening of conditions like asthma and heart disease. But the long-term risks are equally concerning. Studies show that wildfire smoke can contain harmful compounds that linger on indoor surfaces, potentially prolonging exposure even after the smoke event ends.

Health officials advise people to stay indoors during smoke events—but staying inside is only half the battle. The smoke doesn't stay outside. It infiltrates through doors, windows, vents, and even tiny cracks in your home's structure. According to the EPA, indoor PM2.5 levels during wildfire events typically range from **55% to 60% of outdoor levels** when doors and windows are closed and portable air cleaners aren't in use.

**The good news: you can dramatically improve the air inside your home with a few strategic steps.** Here's how.

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## Step 1: Seal Your Home Against Smoke

### Close Windows and Doors—But That's Not Enough

The first and most basic step is to close all windows and doors to keep smoke out. But this alone won't protect you. Studies show that even with doors and windows closed, particle levels indoors can range from as low as 30% to as high as 100% of outdoor levels, depending on your home's construction.

Newer homes and homes with air conditioning tend to keep ambient air pollution out more effectively than older, draftier homes. If your home is older, pay special attention to gaps around doors and windows. Use weather stripping or tape to seal visible gaps if necessary.

### Adjust Your HVAC System

If you have central air conditioning, it can be one of your most powerful tools for cleaning indoor air—**if you set it up correctly**.

**The critical steps:**

1. **Replace your filter** with the highest level of filtration your system can handle. **Filters rated MERV 13 or higher are ideal.** MERV 13 filters are designed to capture the fine particles found in wildfire smoke.

2. **Set the air to recirculate.** Close any vents that pull air from outside. If your system has a fresh air intake, close it or set the system to recirculate mode.

3. **Run the fan continuously.** The University of Oregon's Center for Wildfire Smoke Research and Practice recommends **running your system's fan through the entire smoke event, not just when cooling.** Switching your thermostat from "Auto" to "On" ensures the air is constantly being filtered.

4. **Check the filter often.** Replace it when it appears dirty—dark brown or black is a clear sign.

**The impact:** Upgrading to a high-efficiency MERV 13 filter can reduce PM2.5 concentrations by about 50% when running the system fan continuously. Even with a typical low-efficiency filter, running the system continuously may reduce particle concentrations by about 24%.

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## Step 2: Use a Portable Air Purifier (and Know What to Look For)

If you don't have central air—or even if you do—a portable air purifier can make a significant difference.

### The HEPA Standard

**Only HEPA air cleaners are designed to remove the fine particles found in wildfire smoke.** A true HEPA filter captures **99.97% of particles down to 0.3 microns**—the size range that matters most during a smoke event. These particles are the ones that can penetrate deep into your lungs and cause serious health problems.

**Beware of "HEPA-type" filters.** They're not the same as true HEPA, and they won't provide the same level of protection. When shopping, look for the words "true HEPA" on the packaging.

### What Else to Look For

- **Activated carbon filters** work alongside HEPA filters to remove odors and gases, which HEPA filters alone can't capture.

- **No ozone generators.** Ensure the purifier does not generate ozone, another harmful air pollutant.

- **Size it correctly.** A purifier that's too small for your room won't be effective. Check the manufacturer's recommended room size.

- **Higher fan speed matters.** Many purifiers achieve their best results only on the highest fan setting. Run it on high as much as possible during a smoke event.

### Top-Rated Purifiers for Wildfire Smoke

Recent testing by Consumer Reports and other experts has identified several standout models:

| Purifier | Best For | Key Feature |

|----------|----------|-------------|

| **Coway Airmega ProX** | Extra-large rooms (up to 2,126 sq ft) | Excellent smoke removal; quiet for its size |

| **Blueair Blue Pure 211+ Max** | Medium-large rooms (540 sq ft) | Strong smoke removal; lightweight |

| **Blueair Blue Pure 311i+ Max** | Medium rooms (465 sq ft) | Carbon filter targets wildfire smoke; portable |

| **Shark NeverChange Air Purifier Max** | Large rooms | Long-lasting filter |

| **Honeywell HPA120W** | Small rooms (150 sq ft) | Affordable; low operating cost |

**Cost considerations:** Portable air purifiers range from roughly $75 to $800. Annual operating costs (filter replacements + energy) can range from about $65 for smaller units to over $250 for larger, more powerful models.

### The "Clean Room" Strategy

If you can only afford one portable air filter, use it to create a **"clean room"** in your house—a room where you can spend most of your time during the smoke event. Choose a room with:

- No fireplace

- As few windows and doors as possible

- A door that can be kept closed to isolate it from the rest of the house

Run the purifier continuously in this room, and spend as much time there as possible—especially during sleep.

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## Step 3: Build a DIY Box Fan Filter—It Really Works

**Here's the best-kept secret in wildfire smoke protection:** You can build an effective air purifier for a fraction of the cost of a commercial unit.

### The Evidence

Scientists at the Environmental Protection Agency have found that DIY filters made by duct-taping furnace air filters to a box fan are **just as good at removing fine particles of pollution from the air as a small commercial air purifier.**

Puget Sound Clean Air Agency tested this method with air monitors and found that a box fan filter **reduced airborne particulate matter dramatically.**

"The good news is they really work," said Amy Kalkbrenner, an environmental epidemiologist at the University of Wisconsin-Milwaukee.

### How to Build a Simple DIY Filter

The simplest design requires just two supplies:

- **One 20-inch box fan**

- **One 20x20x1-inch MERV 13 furnace filter**

**Instructions:**

1. Identify the **back** of the fan—the side where air flows inward.

2. Place the filter against the back of the fan.

3. Make sure the **arrow on the filter points toward the fan.**

4. Secure the filter with **tape or bungee cords.**

### How to Build a Corsi-Rosenthal Box (More Powerful)

For even better performance, build the "Corsi-Rosenthal box" design:

**Supplies:**

- One 20-inch box fan

- Four 20x20-inch MERV 13 filters

- Cardboard

- Duct tape

**Instructions:**

1. Tape the four filters together in a cube shape (without the bottom).

2. Place the box fan on top of the cube, with the fan blowing **outward** (away from the cube).

3. Tape the fan to the filters, creating an air-tight seal.

4. Make sure the **arrows on the filters point inward**—toward the center of the cube.

### Important Tips

- **The fan should blow out**, not suck air in through the filter.

- **Make sure everything is taped up air-tight.** Air leaks reduce effectiveness.

- **The arrows on the filters should point in** (toward the fan or the center of the cube).

- You can modify the design to use one or two filters instead of four if you're on a tight budget.

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## Step 4: Don't Make the Air Worse

During a smoke event, your goal is to keep indoor air as clean as possible. That means **avoiding activities that create additional pollution.**

### What NOT to Do

- **Do not smoke or vape indoors.**

- **Do not burn candles.**

- **Do not use gas, propane, or wood-burning stoves or fireplaces.**

- **Do not use aerosol sprays.**

- **Avoid cooking methods that create smoke—especially frying and broiling.**

- **Avoid vacuuming.** (It can stir up particles that have settled on surfaces.)

### The Heat Trade-Off

When smoke and heat occur together, the impacts on your health are even greater than when they occur by themselves. If it's too hot to stay inside with windows closed, seek shelter elsewhere.

**The ideal scenario** is running your AC unit and using an air purifier or a DIY box fan filter to clean the air at the same time.

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## Step 5: Watch the Air Quality Index (AQI)

The Air Quality Index (AQI) is a tool that tells you when you should take action to reduce your smoke exposure.

- **101-150 (Unhealthy for Sensitive Groups):** Sensitive individuals should limit prolonged outdoor exertion.

- **151-200 (Unhealthy):** Everyone should limit prolonged outdoor exertion.

- **201-300 (Very Unhealthy):** Everyone should avoid prolonged outdoor exertion.

- **301-500 (Hazardous):** Everyone should avoid all outdoor activity.

Many weather apps and websites provide real-time AQI data for your location. Check it regularly during wildfire season.

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## What About Masks?

N95 and KN95 masks can offer effective protection from the fine particulate matter (PM2.5) in wildfire smoke.** However:

- **N95s cannot protect against the gases** in wildfire smoke (like carbon monoxide), which may also cause irritation.

- **Medical masks and cloth masks don't offer the same protection** against smoke and are not recommended for wildfire smoke exposure.

- **It is not safe to wear an N95 while sleeping.**

- **Wearing an N95 may make breathing more difficult.**

**The bottom line:** Masks are most useful when you need to go outside briefly, but they are not a substitute for clean indoor air.

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## Key Takeaways

| Action | Why It Matters |

|--------|----------------|

| **Close windows and doors** | Keeps outdoor smoke from entering |

| **Run AC with MERV 13+ filter** | Traps fine smoke particles |

| **Set AC to recirculate** | Stops drawing smoky outdoor air in |

| **Run fan continuously** | Constantly filters air, not just when cooling |

| **Use HEPA purifier in a "clean room"** | Removes particles from one room |

| **Build a DIY box fan filter** | Affordable, effective alternative |

| **Avoid smoking, candles, frying** | Prevents adding more pollution indoors |

| **Watch the AQI** | Know when to take action |

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## Conclusion: Something Is Better Than Nothing

Wildfire smoke is becoming an annual reality for more Americans every year. The health risks are real, but so are the solutions.

"The basic message is that something is better than nothing," said Amy Kalkbrenner, an environmental epidemiologist at the University of Wisconsin-Milwaukee.

Whether you invest in a high-end HEPA purifier, build a DIY box fan filter, or simply upgrade your HVAC filter and run your fan continuously, **every step you take makes a difference.** Your lungs—and your future self—will thank you.

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## Disclaimer

**IMPORTANT:** This article is for informational and educational purposes only and does not constitute medical, environmental, or professional advice. Air quality conditions, health risks, and product effectiveness can vary based on individual circumstances. The EPA, CDC, and local health authorities provide the most current guidance during active wildfire events. If you experience severe respiratory symptoms, contact a healthcare provider immediately. Always consult product manuals and safety guidelines when using air purifiers or DIY filtration devices. The author does not endorse specific products or brands mentioned in this article.