As we gear up for the holiday season, it’s essential to prioritize the quality of the air we breathe indoors. This can significantly affect our wellbeing, especially during gatherings.
By Jeff Masters, who served as a hurricane scientist with the NOAA Hurricane Hunters from 1986 to 1990. He co-founded the Weather Underground in 1995, acting as its chief meteorologist and a Board member until 2012. This article was originally published at Yale Climate Connections.
We’ve all experienced those moments in quiet settings—whether in church or during a lecture—when the urge to doze off suddenly overwhelms us, even with a good night’s sleep. Such drowsiness can often be traced back to one culprit: inadequate indoor air quality, primarily due to a rise in carbon dioxide levels from poor ventilation.
Beyond simply causing fatigue, low air quality can hinder productivity, lower academic performance, and facilitate the spread of illness. Fortunately, simple measures such as changing furnace filters and utilizing air purifiers can greatly enhance our indoor environment, improving both health and overall quality of life.
The Health Risks of Poor Air Quality and Ventilation
Current outdoor carbon dioxide levels hover around 427 parts per million (ppm), but this figure can surge in enclosed spaces. Elevated CO2 levels negatively impact brain function, with children being particularly susceptible. For instance, CO2 concentrations exceeding 1,000 ppm typically lead to increased sleepiness and reduced cognitive performance. In one study, participants exposed to 1,000 ppm of CO2 experienced a 15% decline in test scores, which plummeted by 44% at 2,500 ppm. High CO2 levels effectively impair cognitive abilities!
Once CO2 reaches 2,000 ppm, symptoms can escalate to include headaches, fatigue, poor concentration, rapid heartbeat, and nausea. Therefore, maintaining lower CO2 levels via better ventilation is crucial.
I regularly use a portable CO2 monitor, and I’m often astonished by the elevated CO2 levels in various buildings—ranging from 2,000-3,000 ppm in places like restaurants, medical offices, meeting rooms, churches, and airplanes. Given that climate change is likely to push more individuals indoors to escape rising temperatures and wildfire smoke, ensuring quality indoor air is more important than ever.
If you haven’t listened to this yet, I highly recommend it. All your basic questions about indoor air quality — what are the pollutants? what tech can keep them out? what policies do we need? — are answered.
— David Roberts (@volts.wtf) November 22, 2025 at 9:23 PM
Regrettably, there are currently no national regulations in place that dictate maximum CO2 levels in buildings. Dr. Georgia Lagoudas from Brown University highlights that the only ventilation requirement in the U.S. is that 80% of occupants should not complain about odors. Because CO2 is both odorless and invisible, buildings can legally harbor health-harming levels of CO2 alongside other volatile organic compounds (VOCs) without alarming anyone as long as no unpleasant smells are present.
The American Society for Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) drafts voluntary building codes, including indoor air quality standards, which are only adopted at the state and municipal levels. Unfortunately, many buildings were designed to be more airtight during the energy crisis of the 1970s, sacrificing ventilation for better energy efficiency. This change has led to increased instances of sick building syndrome, where poorly-ventilated environments result in occupants feeling unwell due to high CO2 accumulation.
Moreover, the majority of respiratory pathogens, including COVID and the flu, are airborne. They spread more readily in poorly ventilated areas. CO2 levels can indicate the amount of air in a room that comes from exhaled air. At 800 ppm, about 1% of the air we breathe is exhaled by others. At 2,000 ppm, this rises to 4%, indicating that if someone in a crowded indoor space has a respiratory illness, the risk of transmission increases significantly.
Recently, there has been a notable push to enhance indoor air quality, as people spend upwards of 90% of their time indoors. Affordable measures can yield substantial impacts on health. In September, the Global Pledge for Healthy Indoor Air was initiated at the United Nations, establishing the first international declaration affirming that clean indoor air is a fundamental human right. In the U.S., the Clean Indoor Air Initiative from Brown University states that improving indoor air could potentially reduce respiratory diseases by 80%, decrease student absences by 13%, enhance workplace productivity by 11%, and generate an annual economic benefit of $23 billion through a 35% reduction in sick leave.
Monitoring CO2 and PM 2.5 Levels for Safety
Investing in a portable CO2 monitor can be a wise decision for assessing your indoor air quality. For example, Boston Public Schools actively monitor CO2, CO, and PM2.5 levels in every classroom. Numerous commercially available devices exist, and one favored by professionals in the field is the Aranet4, which is priced at $179 on Amazon.
A commendable low-cost option is the INKBIRD PTH-10C, priced under $40 (see the review here). Additionally, my spouse has assembled a budget-friendly home CO2 sensor using Arduino technology for about $30 (instructions available here). This gadget plugs into a USB battery for portability, and there’s a free app called Sensirion MyAmbience that displays the collected data.
Ikea is set to release their Alpstuga air quality sensor, measuring CO2, PM2.5, temperature, and humidity, priced at just $32, though its availability in North America hasn’t been announced yet.
Great overview of CO2 monitoring. High CO2 is associated with COVID spread and causes sleepiness, headaches, fatigue, and lower test scores (15% drop at 1000 ppm; 44% drop at 2500 ppm). I’ve found 2000-3000 ppm in restaurants, doctor offices, meeting rooms, churches, and airplanes.
— Dr. Jeff Masters (@drjeffmasters.bsky.social) December 5, 2025 at 12:52 AM
It is also important to regularly monitor and minimize your indoor PM2.5 levels. Globally, over 4 million deaths each year are linked to outdoor PM2.5 air pollution, as reported by the World Health Organization.
Yet, the true impact may be even higher. A 2019 study found that PM2.5 and ozone pollutants were responsible for approximately 8.8 million premature deaths globally, with the majority stemming from PM2.5. Furthermore, a 2021 investigation revealed that fossil fuel emissions led to about 8 million deaths annually. Indoor PM2.5 pollution, mainly from burning low-quality fuels for cooking, accounts for an additional 3.2 million annual deaths. Compromised air quality typically results in premature deaths occurring about 12 years earlier than expected.
The Health Effects Institute’s 2018 study indicates that PM2.5 pollution contributes to around 87,000 premature deaths yearly in the U.S., affecting various health conditions from cardiovascular disease to lung cancer and dementia. Yet other analyses estimate this number to be much higher: a 2019 study suggested PM2.5 accounted for 197,000 deaths, while a 2018 investigation estimated a toll between 230,000 to 300,000 deaths, with 40-60% attributed to indoor PM2.5 contamination, typically from unfiltered outdoor air.
Implementing HEPA filtration is an efficient way to eliminate PM2.5 particles. In order to keep indoor PM2.5 levels below an AQI of 50, acquiring a PM2.5 monitor is recommended. Breathesafeair.com offers valuable reviews, along with the South Coast Air Quality Management District, which maintains a comprehensive database comparing the performance of popular home monitoring devices to expensive official instruments. My portable Temtop PM2.5 sensor (currently priced at $60 on Amazon) effectively monitors PM2.5, VOCs, and formaldehyde. While the Temtop has been helpful in indicating when to ramp up HEPA usage, I have found it occasionally gives erratic high AQI readings.
For outdoor conditions, I would recommend the PurpleAir sensor, which I’ve trusted for over ten years (see my review here).
Monitor your indoor PM 2.5 levels to understand your filtration and ventilation needs. The South Coast AQMD tests how well the most popular home monitoring devices perform compared to expensive official units: www.aqmd.gov/aq-spec/sens…. And this site has some super-detailed reviews:
— Dr. Jeff Masters (@drjeffmasters.bsky.social) December 11, 2025 at 12:14 AM
Five Strategies to Enhance Indoor Air Safety
1. Regularly Replace Your Furnace Filter
Furnace filters capture small particles (including viruses) but do not lower CO2 levels. Consult your HVAC technician to ensure the filters in all buildings are regularly updated. During periods of heavy Canadian wildfire smoke, we need to change our filters more frequently, as smoke clogs them more rapidly than regular dust.
The effectiveness of filters in trapping airborne particles is measured by the Minimum Efficiency Reporting Value (MERV) scale. A higher MERV number indicates better performance. ASHRAE suggests using at least a MERV-13 filter, which is 85% effective at capturing particles sized 1 to 3 microns—this includes dust, wildfire smoke, pollen, and respiratory aerosols carrying viruses. A MERV-14 filter achieves at least 90% effectiveness with the same range of particles.
In contrast, a standard MERV-8 filter is only around 20% effective at capturing these particles. Research indicates that using MERV-13 filtration can reduce the chances of contracting COVID by 42% compared to MERV-8 filtration. Nevertheless, many older ventilation systems may be restricted to MERV-8 filters since their fans struggle to push air through the denser MERV-13 types without suffering damage.
Tip: Boost your COVID protection simply by running your ceiling fan. Doing so circulates the air, dispersing virus particles into the walls, where they are more likely to be captured and removed.
2. Use Air Purifiers
Investing in plug-in air purifiers is one of the simplest ways to enhance your indoor air quality.
I steer clear of units using UV light for air treatment, as they can create ozone, which is harmful to lung health. While properly designed UV air purifiers can minimize ozone emissions, the inconsistency in independent testing makes such models less reliable. Instead, I recommend traditional “high-efficiency particulate air” (HEPA) units, which utilize pleated filters with fans. HEPA filters notably reduce COVID risk and lessen the health impacts of PM2.5 during wildfire events. Always operate purifiers at the highest noise level you can tolerate, as proper air mixing is vital.
HEPA units come with a black pre-filter, which contains activated charcoal (replace every three months for optimum VOC absorption) and a pleated white main filter (replace every 6-12 months). You might be able to extend the life of the black pre-filter by brushing off accumulated particles. Adjust the filter replacement intervals based on usage, fan speed, and air quality conditions. If you have pets that shed, consider a HEPA unit featuring a pre-filter designed for frequent cleaning. Note: HEPA filtration does not eliminate CO2 levels—it strictly reduces airborne particles (including viruses).
If investing $150-$900 in a commercial HEPA unit isn’t feasible, consider constructing your own using a box fan, duct tape, and MERV-13 filters. These DIY solutions are effective and are known as Corsi-Rosenthal boxes. The EPA and various universities have tested these, confirming equal performance to high-end commercial HEPA units.
My spouse has crafted a four-filter Corsi-Rosenthal box for around $75, utilizing a 20-inch Lasko Windmachine fan ($49 on Amazon) and a four-pack of 3M Filtrete MERV-13 or MERV-14 filters ($52 at Costco during sales). She has also fashioned a two-filter Corsi-Rosenthal box using a nine-inch Black and Decker fan, which costs around $18 on Amazon, and some battery-operated portable fans employing 12-volt computer fans for use in our tent during wildfire seasons. Homemade Corsi-Rosenthal boxes can effectively operate for about a year; after that, check if filters appear dirty and replace as needed (retain the fan and just acquire new filters). Cleanaircrew.org and UConn Indoor Air Quality Initiative provide detailed instructions for building Corsi-Rosenthal boxes. Don’t skip making the cardboard shroud; it boosts the system’s efficiency.
Figure 2. I measured a CO2 concentration of 3,001 ppm in this church – a level high enough to cause headaches and significant spread of respiratory illnesses. The church was running one HEPA unit with a clean air delivery rate of about 100 cubic feet per second. According to ASHRAE Standard 241-2023, a CADR of 3,600 cubic ft/s would be required for adequate infection control in a room with that level of CO2 and 35 people singing. This implies the need for an additional 35 HEPA units or four large four-filter Corsi-Rosenthal boxes (though the noise would be considerable). However, this additional HEPA filtration would not reduce the CO2 level. The church’s ventilation system needs upgrading.
To calculate how many HEPA or Corsi-Rosenthal units are required for a room with a specific CO2 level, you should determine the clean air delivery rate, or CADR, of the units. My spouse has developed a CADR calculator to help you understand how much filtration is necessary for effective infection control, as described in ASHRAE Standard 241-2023. This CADR calculator includes a CADR HEPA spreadsheet for the U.S., indicating that our favorite HEPA unit, the Coway Airmega AP-1512-HH “mighty” ($230 on Amazon), operates at the highest fan speed (3) with a CADR of 253 cubic feet per minute, albeit somewhat noisily (64 dBA). Meanwhile, an AirFanta 3Pro delivers a CADR of 413 cubic feet per minute at 59 dBA for around $160 on Amazon (see detailed review here). Conversely, a homebuilt Corsi-Rosenthal HEPA unit can provide a CADR of 900 cubic feet per minute at medium speed (though it requires more space and generates more noise).
If you are utilizing a four-filter Corsi-Rosenthal box, you can refer to the following guidelines from the Corsi-Rosenthal Foundation to assess your filtration needs dependent on your space.
How many CR Boxes do you need for a particular space?
Check out our new handy charts to help gauge your needs for:
– school settings
– residential settings
*Notice the guidance is different based on your goals:
– mitigating wildfire smoke
– mitigating bioaerosols
— Corsi-Rosenthal Foundation (US) (@crfoundationus.bsky.social) October 9, 2025 at 5:22 PM
3. Invest in an Energy Recovery Ventilator
Boost your space’s ventilation by acquiring an Energy Recovery Ventilator (ERV). The installation of a professional-grade unit typically costs $2,000 and above for homes, but the investment is recouped over time through improved air quality and energy savings. ERVs draw in fresh air from the outside while expelling stale indoor air, using the heat from the outgoing air to warm incoming air. The CDC suggests a minimum of five air exchanges per hour to reduce COVID risks. Some ERVs are available for DIY installation without professional assistance.
4. Consider Replacing Your Gas Stove
Gas stoves pose a serious hazard to indoor air quality and are of public health concern, as stated by the American Public Health Association. The association also warns that gas stoves increase the risk of childhood asthma; recent studies have linked gas stoves to almost 13% of childhood asthma cases in the U.S. Additionally, gas stoves emit hazardous levels of three pollutants regulated by the EPA: CO, NO2, and PM2.5, along with benzene, a known carcinogen. A recent investigation suggested that high gas stove usage, without adequate ventilation, increases cancer risk due to benzene emissions. Furthermore, gas stoves can release CO2 levels exceeding 1,000 ppm—while this doesn’t contribute to COVID risk, it can cause headaches and impair cognitive function.
Research indicates that the U.S. EPA’s “safe” limit of 100 for CO, NO2, and PM2.5 (labelled “Unhealthy for Sensitive Groups”) is surpassed for an average of 99 minutes daily when using gas stoves. The NO2 emissions are particularly detrimental. If you cook with a gas stove, ensure the overhead ventilation fan is turned on and windows in the kitchen are open to facilitate better airflow.
If you operate a gas or propane stove (38% of homes in the U.S. do), think about purchasing a single-burner induction stove to minimize gas cooking (cost: approximately $70; you’ll also need steel or cast iron cookware). According to Rewiring America, a full four-burner induction stove/oven costs about $1,300 and may require dedicated wiring and electrical upgrades. Some newer induction stoves can operate on high-capacity batteries, costing around $6,000-$7,000, which eliminates the need for costly electrical enhancements.
Gas stoves play an outsized role in Americans’ exposure to harmful nitrogen dioxide pollution, a new study finds.
— Inside Climate News (@insideclimatenews.org) December 10, 2025 at 6:45 PM
5. Use N95 Masks and Conduct COVID Testing
Though former President Biden declared the COVID pandemic over in September 2022, the U.S. has experienced several hundred thousand COVID-related deaths since then, with my own father being one of them.
The long-term societal costs of COVID will be astronomical when taking into account the fatalities, health complications from long COVID, and unforeseen medical expenses. Our casual approach to COVID mirrors how we address the climate crisis—it’s deeply concerning.
I continue to regard COVID as a serious threat and have consistently worn a snug-fitting N95 mask during indoor functions since the pandemic began—even while dining (it’s simple to slip it aside briefly). Remarkably, I have yet to contract the virus. Wearing a mask is a minor inconvenience that effectively shields you from one of modernity’s significant health challenges. Studies affirm that masking does not significantly impact CO2 levels; it merely extends your airway by a few centimeters, which doesn’t cause significant variation.
For a COVID-free indoor gathering, it’s vital that everyone wears an N95 mask. I prefer the 3M VFlex 9105 ($42 for a pack of 50 on Amazon). I’ve comfortably worn one for 12 hours during long flights. While surgical masks offer around 80% effectiveness against the virus and are generally more comfortable, consider providing a box of free masks at your gathering to encourage usage. Some high-quality surgical masks feature a flexible wire for a better fit and are available for just 17 cents each when purchasing in bulk.
Testing for COVID is a routine part of my life. I attended a wedding three years ago where everyone tested (70 individuals) before the event, and it was successful: no one got sick. However, at a wedding last year where no testing protocol was followed, we witnessed a superspreader event with over 30 illnesses. Despite my wife wearing an N95 mask indoors and being fully vaccinated, she contracted the virus, likely from an outdoor gathering with light wind. This brings to light that close proximity still carries transmission risks, regardless of ventilation and filtration quality. Unfortunately, my wife now copes with COVID-induced heart ailments.
Hear how frequent illness and poor air quality posed major problems for students at Red Rock Elementary, and how Corsi-Rosenthal Boxes boosted their health and ability to learn.
Learn more at:
CleanAirK12.com
CorsiRosenthalFoundation.org
#CRBox #CleanAirK12
Full Video:
youtu.be/dUYLPUfkv0g
— Corsi-Rosenthal Foundation (US) (@crfoundationus.bsky.social) February 25, 2025 at 12:25 AM
Additional Resources on Air Quality from Yale Climate Connections
15 sources of wildfire smoke forecasts for North America (2025)
Dealing with wildfire smoke indoors? Create a DIY Corsi-Rosenthal air filter (2023)
Alejandro Daly: From illness to advocacy for clean air (2023)
Communities banding together for low-cost air monitoring (2024)
The detrimental health effects of gas stoves (2023)
Understanding the health impacts of wildfire smoke (2021)
