Part 2: Indoor Air Quality

This blog is a follow up to my last article, Making Sense of Air Quality which focuses on outdoor air quality. While outdoor air quality is important, indoor air quality also plays a crucial role in promoting health. All of us take shelter during this wildfire season for refuge from the unrelenting smoke. However, in the spring and summer months those who suffer from allergies will often take similar refuge from the pollen.

PurpleAir tracks both indoor and outdoor air quality. A simple query from Siri pulls air from the nearest high resolution EPA air sensor. If we look at PurpleAir, we can see much more data about the air quality in the area.

If you look closely, you’ll see some green values in a sea of red. These outliers are extremely important. PurpleAir sells indoor and outdoor air quality sensors. The map below on the left shows outdoor air quality only. The map below on the right only shows indoor air quality. The legend in the lower left of all three graphs shows which sensors appear on the map.

If we look at the map below on the left, we can see more consistency The legend in the lower left has all of the indoor sensors removed. The indoor map shows wide variation in air quality.

Indoor air quality can vary wildly from the ambient outdoor air quality. Factors that can affect indoor air quality include the airtightness of the house, the filtering efficiency of the HVAC system, and the members/activities within the home.

Should my home be air tight?

Good houses have to balance how airtightness and breathability. A well-insulated, airtight home will prevent drafts on cold winter days, keep out wildfire smoke in the fall, and reduce pollen in the spring. Making sound investments in windows, weather stripping on doors, closing fireplace flues, and insulation within the home will improve its airtightness and energy efficiency.

Houses can’t be completely airtight. Why? Bathroom fans and kitchen vents will suck air from within the home and push it out into the atmosphere. This removes excess steam (prevents mildew) and cooking smoke (prevents staining) and releases it into the atmosphere. As the fan vents air into the atmosphere, outside air will creep into the home through various cracks to replace the vented air. When the air is polluted outside, running these fans can decrease indoor air quality.

The role of the HVAC system

When I moved into my first place, the landlord told me to change the air filter in the heater at least once per quarter. Dan, meet HVAC system! HVAC stands for heating, venting, and air conditioning. It’s the critical part of your home that keeps it cool in the summer and warm in the winter. HVAC systems pull air through a central vent and redistribute it throughout the house, ensuring even heating and cooling.

All HVAC systems have an air filter that will clean the air as it passes through the system. All air filters are not created equal. When it came time to change my air filter in my home, I went to Home Depot and got the cheapest thing to fit the landlord’s requirement. In other words, I didn’t really know what I was buying. Each HVAC system will take a specific air filter dimension. Take note of the length, width, and depth of your air filter.

The other important metric is the filtration quality of the filter. Each filter sold in the United States has a MERV value, which indicates its filtration quality. MERV is short for Minimum Efficiency Reporting Value. A higher MERV rating indicates a better filter and usually a higher price.

Let’s take a look at how MERV rating compares against the primary pollutants measured by PurpleAir sensors.

MERV RatingPollutant TypeCommon examples
1-8PM > 10Hair
Dust Mites
Textile and carpet fibers
9-10PM < 10Pollen
Mold Spores
Dust
Hair Spray
Plant Spores
11-12PM < 2.5Combustion particles
Organic compounds
Insecticide dusts
Pet dander
Airborne metals
Smog
13-16PM 1Bacteria
Aerosols
Smoke
Cooking Oil
Fine grit smog

Investing in a high-quality MERV 13 air filter goes a long way in ensuring good air quality within the home. The wildfire smoke in Northern California is far from campfire smoke. The fire is burning trees, but it’s also burning metals, glass, paint, and many other man-made materials. The more your HVAC system can filter those components out, the better your indoor air quality will be.

Changing your air filter

Changing your air filter will keep the air in your home fresh. It will also maintain your HVAC system’s efficiency as the fan will not have to labor to pull dirty air through the filter. Your HVAC system is a lot like a giant vacuum cleaner. As a vacuum gets more dirt inside of the bag the suction decreases. Likewise, as the filter gets more dirt inside of it, the HVAC system works harder pulling air through a dirty filter, making the system less efficient. 

In addition, many smart thermostats like the Google Nest will remind you to change your air filter. Take advantage of these features. During wildfire season, change your air filter more frequently. I change mine every month or two depending on the smoke intensity. Here is my air filter after two months of use (above) compared to a new filter (below). One of those months was during the first wave of wildfires.

ProTip: I get my filters from US Home Filter. They automatically ship filters once per quarter to keep my home’s air fresh.

Remember these two small tips when installing your air filter. First, write installation date with a permanent marker on the filter itself. Make sure to include the month, date, and year as time always passes faster than we think. Secondly, many air filters are directional. Make sure that the air flows across the air filter in the direction indicated on the filter.

Do I really want to be breathing all of that dirt? No!

Check your vacuum!

Activities within the home can also contribute to poor air quality. One culprit is vacuuming. Vacuuming disrupts dirt and can release it back into the air.

When vacuuming, always use a sealed, HEPA grade vacuum cleaner. Many manufacturers will claim HEPA, but their machines continue to leak dirt. If your vacuum is dirty near the exhaust, it’s likely not performing as it should. Make sure to use quality HEPA bag and filters. Also, ensure all of the seals remain tight on the machine.

Cooking can also reduce air quality within the home. Balance using the venting systems when air quality is extremely poor outside

Bringing fresh air in!

Sometimes, air quality is excellent outside! Take advantage of those opportunities. I love using my whole house fan to quickly cool the house after the sun goes down on a warm summer day. Likewise, I love blowing out stale winter air when the mercury bumps above 70° outside. Whole house fans significantly move massive amounts of air throughout the home and out the attic vents, cleaning stale air and reducing the HVAC system load.

Your home’s windows, doors, HVAC system, and fans all work together to maximize air quality within the home. Please share in the comments any additional tips you’ve learned!

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