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Knock out the NOx

Knock out the NOx

This title may sound like an ad for high-octane gasoline, but I’m talking about air pollution here!  In air purification “PM” or particulate matter often steals the show and drives the decisions behind purchasing this filter versus that filter and this purifier over that one.  But what about the gasses in air pollution?  Gasses are not particulates–they are harmful molecules in the air that cannot be filtered out by plain HEPA filters.  So what are they and how can we get rid of them?

NOx is one term for two nitrogen gasses commonly found in urban areas.  It includes nitric oxide (NO), which is a colorless, odorless gas, and nitrogen dioxide (NO2), which is a reddish-brown gas with a pungent odor.  They are produced during combustion: in factories, in transportation vehicles, and even boilers for heating apartments, office buildings, hospitals, universities, etc.  They’re also produced indoors by gas stoves and furnaces when they are not properly vented. 

Nitric oxide in ambient conditions is not harmful, as it dilates (relaxes) blood vessels and improves oxygenation.  However in higher concentrations, it does two things: it will create a burning sensation in your throat and chest as it changes into nitric and nitrous acid, and it goes deep into your lungs to react with blood cells and eventually be excreted by the kidneys.   Nitric oxide in the air that is not breathed in is converted to NO2, the other dangerous nitrogen gas, or precipitated in moisture as acid rain.  (Poison Facts: Low Chemicals: Nitric Oxide)  

According to Clarity.io, a manufacturer of air monitoring sensors, nitrogen dioxide pollution stays mostly concentrated in the area where it is emitted, meaning that areas with high vehicle traffic like urban areas tend to have the highest NO2 levels. (Clean air cities: Innovative approaches to improving air quality in urban settings)  NO2 is a pretty nasty gas: it causes inflammation of the respiratory pathways, worsened cough and wheezing, reduced lung function, increased asthma attacks and is likely to be a cause of asthma in children. (Nitrogen Dioxide)  

If you’ve been reading our articles or have any experience with air pollution or wildfire smoke, you’ll know that outside pollution eventually makes it into our homes, because they’re leaky.  Thus, we don’t have a “safe place” to get away from air pollution unless we leave the city or take steps to make our home air cleaner.  As we already discussed, just buying a plain HEPA filter or making a DIY air filter (Corsi-Rosenthal box) will not take NOx out of your home.  There are ways to get rid of NOx, however: upgrade to a HEPA filter with activated carbon, AND eliminate the sources of NOx inside your home.

A 2014 study conducted in Baltimore, MD found that a two-pronged approach really helped reduce the NO2 in homes:  

  1. Replacement of unvented gas stoves with electric stoves reduced NO2 concentrations by 51% and 42% in the kitchen and bedroom, respectively, indicating that stove replacement impacts NO2 concentrations beyond the kitchen (even when the home also has a gas furnace or drier).

  2. Placement of air purifiers with HEPA and carbon filters in the home results in a nearly 27% decrease in median kitchen NO2 concentrations immediately (1 week after placement), and reductions were maintained at 3 months following intervention. 

Although the study also included the addition of a ventilation hood in some homes, it was unclear whether the hood helped lower NO2 emissions (ventilation does help remove air pollution, but it depends on strict use of the hood during and after cooking).  

Activated carbon adsorbs the NO2 and secures it in the filter, until the filter is changed.  Adsorption of NO2 can be enhanced by 38-55% by adding coatings to the activated carbon, such as potassium hydroxide (KOH).  (Development of an activated carbon filter to remove NO2 and HONO in indoor air)  Even wildfire smoke has NO2 in it; according to this Canadian government environmental webpage, wildfire smoke is a complex mixture of gases, particles and water vapour that contains pollutants such as: sulphur dioxide, nitrogen dioxide, carbon monoxide, volatile organic compounds, fine particulate matter (PM2.5), and ozone.  Activated carbon can handle these gasses and VOCs, while HEPA filtration is used against PM2.5

Therefore, city dwellers, take note!  Whether it’s “business as usual” as pollution from downtown traffic penetrates your home, or wildfires hundreds of miles away turn the skies orange and hazy, a purifier with HEPA and activated carbon is your best bet to knock out the NOx.  Be on the lookout for purifier giveaways, too!   Here are some programs being offered at this time:

  • San Joaquin Valley Air Pollution Control District (California) is giving away air purifiers to keep residents safe from air pollution.  If you live in the area, you can apply for one here

  • The City of Philadelphia is giving air purifiers to early-childhood education centers and schools.  You can learn more and link to an interest form in this article.

  • Project N95, a national non-profit working to protect people and their communities during the COVID-19 pandemic and beyond, has begun a donation pilot program of air purifiers for schools and other organizations.  You can fill out their form here.

Keep your eyes and browser open, as other organizations may spontaneously donate during events like the Canadian wildfires (the University of Connecticut gave away 100 DIY purifiers on June 8, 2023).  During such a time, it’s the neighborly thing to do!

Photo by Jacek Dylag on Unsplash

Negative Pressure Ventilation can be dangerous in your home! Here’s how to prevent it.

Negative Pressure Ventilation can be dangerous in your home!  Here's how to prevent it.

You may have heard the saying “Build Tight, Ventilate Right” and thought, well, that’s for newbuild homes, I can’t go back and seal my house now, it’s been completed for 30 years!  (or 2 or 5 years for that matter).  This is partly true, but we’ve detailed some ways to seal finished homes in our post here.  The thing is, without the Build Tight, you can still get good air quality if you Ventilate Right for your home. 

There are basic “flows” in the universe that are true everywhere, because energy will seek to equalize, meaning move from areas of high energy to areas of low energy.  Heat will move naturally from hot to cold.  Water will flow from high elevation to lower elevation.  A liquid or gas will flow naturally from an area of high pressure to low pressure.  Let’s examine this third example.  In order for air to move in or out of your house, you will need to have an area of high pressure and an area of low pressure.  Otherwise, there will be no air flow!  Sure, wind blowing around the house will naturally generate pockets of high or low pressure, which may act locally on different external rooms.  For moderately sealed homes with all the windows and doors closed, however, the greatest chance for natural airflow to occur is when a chimney damper is open or poorly sealed, and wind flowing across the chimney draws air up out of your house.  Otherwise, no appreciable airflow is going on unless you turn on a mechanical system.  We discussed this in our post “Do Trickle Vents Really Work?”  This lack of airflow can be good and bad.

The good thing is that pollutants from outside, as well as humidity, are not actively flowing into your home when there are no mechanical systems operating.  This is good because you don’t want these things inside, anyway.  Here ends the good. 

The bad is that you are not getting fresh air from the outside (even semi-polluted air can be fresh if it is filtered), and you are not getting bad air from inside (CO2, VOCs and dust) out.  We need active ventilation and it’s mandated for healthy living!  So, we need mechanical systems (fans and the like) to draw fresh air in and get bad air out.

Mechanical systems will always create a pressure differential if they are one-way.  In other words, if you only have an exhaust fan going in your home, it is always pulling air out and there will be a slight negative pressure inside.  If you always have a fan in your open window pointed into the room, it will be bringing air in and pressurizing the room.  If you have fans working in both directions, pressure will be more balanced/neutral, but you will still accomplish the goal of getting fresh air in and bad air out.  In our post “How Does Indoor Air Pressure Affect Ventilation and Air Quality?”  we dove into the pros and cons of different pressure schemes–negative, positive, balanced and balanced with an HRV or ERV, and why we think balanced pressure is the way to go. 

Unfortunately, for most people “ventilation” causes them to think about getting the bad air out only, and this is a problem.  In such homes there is no fresh air supply, turning the house into a stagnant air pool.  When the kitchen and bathroom exhaust fans are turned on or the clothes dryer is started, a slight negative pressure is generated (yep-check out our article on heat pump dryers, where we disclose that regular dryers dump about 200 cfm of conditioned air outside!).  

For an eye-opening demonstration on toxins going into a negative pressure home, check out this video!  

Smoke going down the chimney: 5:45 minutes

Exhaust and fumes from the attached garage: 6:20 minutes

The most dramatic increase in negative air pressure came when the demonstrator closed two interior doors in the simulated house.  The pressure dove from -1.7 paschals to 5.7 paschals!  This is the force that will pull combustion gases, chimney smoke, and garage fumes right into your living space.  Air can be suctioned from cracks in the building perimeter or worse, from combustion gas exhaust flues.  Here are the main culprits that generate negative air pressure:


  • Many people have gas or electric dryers in rooms within their home.  When running a load of laundry, the dryer will suck about 200 cubic feet per minute (cfm) from the conditioned air in your home, and exhaust it outdoors through the vent.  (see our post on heat pump dryers and how this type of dryer eliminates this problem).

  • The loss of 200 cfm of air during a 45 minute-1 hour drying cycle not only wastes money, it creates negative pressure in your home.

  • If you close the laundry room door while operating the dryer, as many people do for noise abatement, most of the negative pressure is confined to the laundry room.  However, this means that:

    • the dryer is being starved for fresh air, which causes a longer drying cycle, and 

    • the laundry room is pulling all kinds of contaminants through cracks and crevices from other places in order to feed the dryer.  Where are many dryers located?  In the garage or next to the garage, or in the basement, which means that you are pulling air from a space that has exhaust fumes and/or mold contamination to dry your freshly-washed clothes.  Yuck!

    • If your laundry room also contains your gas-fired furnace or hot-water heater, it’s likely that the exhaust fumes from the heater are backdrafting to fill the air need for the dryer.  This means that combustion gasses are being sucked into the laundry room and dryer over your freshly-washed clothes.  Danger AND yuck!

  • If you don’t close the laundry room door, the dryer is pulling that 200 cfm of air from the rest of your home.  Add to that your kitchen exhaust range, which is usually 300 cfm or more.   Since the average home is fairly leaky, where is that air coming from?  

    • The attached garage with its car exhaust and fumes from any paints and chemicals stored there: yuck!

    • Down the chimney past a leaky damper: yuck!

    • From the dusty attic through your unsealed attic door and can lights: yuck!

    • From the neighbor’s stinky apartment: yuck!

    • From the pollen-laden or pollution-laden outdoor air through unsealed holes in the wall around your pipes, outlets and switches on exterior walls and other penetrations: yuck!

Gas-burning appliances

If your home uses any type of gas appliance, a negative pressure environment can be very dangerous.  Carbon monoxide can quickly overwhelm people in the home, causing injury or death.  “Backflow” or backdraft is airflow going in the wrong direction; in this case, instead of going out the chimney or combustion gas vent, toxic combustion gases can be sucked into the room where the exhaust fan or dryer is located.  This video from Australia shows how to test for a backdraft situation, and correct it by opening a window.  Here are the steps mentioned:

  • Make sure your gas water heater, furnace or other appliance is shut down and cold.

  • Turn on the exhaust fan(s) and dryer in your home.

  • Hold a smoke pen or candle next to the combustion gas vent.  

    • If the smoke rises straight up or is drawn into the vent, then the exhaust fan is not making enough negative pressure to affect combustion gas venting (good).

    • If the smoke moves away from the combustion gas vent and toward the exhaust fan, then a negative pressure situation is developing.   Open a window until the smoke rises straight up or toward the combustion gas vent.  Take note of how far the window must be opened for this to happen. 

There is another device to help avoid combustion gas backdraft; it’s called a Power Vent for your water heater.  Essentially, a fan in the vent duct of the heater propels gasses outside, instead of just relying on a stack effect.  Typically, power vented water heaters are installed where it’s not possible to vent vertically.  These types of vent systems are less likely to backdraft because of the vent assist (empirestateplumbing.com), but at the same time, they also draw more air from the space, adding to the negative air pressure problem. 


You don’t even have to be using the fireplace to get harmful combustion residues sucked into your home.  If the chimney damper is not closed or well-sealed, air can be drawn down your chimney across layers of soot and ash, bringing it into your living room.  

Kitchen Exhaust Hoods and Bathroom Exhaust Fans

These types of fans are super-helpful to exhaust odorous fumes (and required over cookstoves), but they should be balanced with a filtered intake in order to avoid generating negative air pressure! 

How do I prevent negative pressures inside my home?

To make it all clear, we wanted to summarize what happens unknowingly when certain exhaust appliances are operated, and how to avoid these dangerous situations.

If you only have exhaust fans in your home, open a window to balance that air flow, and keep doors open or install In-Door Return Pathways so that negative pressures don’t increase. 

The best solution for prevention of negative air pressure in our opinion is installation of a Heat Recovery Ventilator (HRV) or Energy Recovery Ventilator (ERV).  These devices can recover the heat or cool in a normal exhaust (like a bathroom vent), and will supply the fresh air needed so that pressure in the home remains close to neutral.  However, it’s not usually recommended to connect the dryer or kitchen exhaust to your HRV or ERV.  Why?  These are typically “dirty” exhausts that would quickly plug up a heat exchanger with lint (in the case of the dryer) or grease (in the case of the kitchen). (Sources: greenbuildingadvisor.com here and here).

In the first part of this Home Diagnosis video, the host shows a kitchen exhaust hood with remote fan and duct silencer (brilliant devices to minimize noise in the kitchen) and preheated and filtered make up air.  Even if you don’t have a preheater/cooler, just having make up air on demand in conjunction with the range hood is a great idea, not to mention it satisfies the International Residential Code (IRC) requirement that “Exhaust hood systems capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be mechanically or naturally provided with makeup air at a rate approximately equal to the exhaust air rate.” (IRC code M1503.4)

The same should go for the dryer.  In order not to pull a suction on the rest of the house, the HRV/ERV should supply fresh air to the laundry room.  

In mild climates where air conditioning or heating is not needed for much of the year, or the homeowner simply likes the idea of opening windows for fresh air, an HRV or ERV doesn’t make much sense.  Thankfully there are ways to get that “makeup air” so that your home air pressure will stay balanced no matter what appliances/vents are running.  Here are some of them:

  • Window screen filters are a great option to get that fresh makeup air by opening a window and passing it through a nanofilter to remove the smallest particles of pollution.

  • In-Door Return Air Pathways by Tamarack Technology bridge the inevitable: closed interior doors.  Simply install these in the bottom of your hollow-core or solid wood interior doors (door must be 1-3/8” thick to fit) so that you can have privacy AND ventilation at the same time.

  • Make up air for the kitchen or balanced kitchen exhaust vents (check out this video for an explanation on how a packaged balanced vent fan works). 

  • This healthy home consultant in NC has a “Clothes Dryer Intake” that supplies make up air to your dryer. 

One last source of negative pressure in the home is power attic roof vents.  Most people think that using a power attic roof vent will cool the attic by sucking out hot attic air, and drawing cooler air in from outside.  The attic does end up being cooler, however, this is not because of outside air being drawn into the attic; the vent is actually so powerful that it is sucking the air-conditioned air from your house up through large and small crevices, into the attic! (bobvila.com) If you have your gas water heater or furnace in the attic, it can also be backdrafting.  In most cases, unobstructed natural vents at the soffit and ridge, combined with a good layer of attic floor insulation, will be better than power attic roof vents.

Just like your budget, don’t let your house go “into the negative” zone when it comes to air pressure, and get that fresh air coming in when you need it.  Balanced or Make Up Air Systems (MUAs) not only avoid pulling all kinds of pollutants into your air space, they also dilute higher CO2 and VOC levels so that you’re feeling comfortable and energized (check out our post on CO2 here).  It’s a decision that can improve your home air quality with only a medium amount of effort and money!