The Only Air Numbers Worth Watching (and Their Targets)

Last updated September 24, 2026 · Monitor My Air

There are really only five numbers that matter for indoor air, and each one has a clear target you can act on. Learn them and you can ignore most of the marketing.

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What it is

Indoor air quality sounds complicated because companies want to sell you complicated products. In reality, a small set of measurements tells you almost everything about whether your air is helping or hurting you. If you learn these five numbers and their targets, you can walk into any room and know what to do. Everything else a fancy monitor shows you is mostly noise.

The numbers that matter

PM2.5 (fine particles): good is under 5 µg/m³, fair is 5 to 15, and poor is above 15. These are the particles that reach deep into your lungs, so this is the single most important pollution number.
CO2 (carbon dioxide): good is under 800 ppm. Above 1000 ppm you get foggy, drowsy, and less sharp. High CO2 is really a sign that fresh air is not getting in, which means everything else is building up too.
Relative humidity: aim for 40 to 60 percent. Above 70 percent invites mold and dust mites. Below 30 percent dries out your eyes, skin, and airways.
VOCs (chemical gases): good is under about 250 ppb. Spikes come from paint, cleaners, new furniture, and air fresheners.
Temperature: around 21°C (70°F) is comfortable for most people and pairs well with healthy humidity.

What NOT to waste money on

You do not need a separate device for every pollutant, and you do not need lab-grade accuracy at home. Ignore any product that promises to measure dozens of exotic chemicals or shows a single mysterious "air score" with no underlying numbers. A score you cannot break down cannot guide a decision. Skip monitors that only work through a subscription app, and be suspicious of anything that constantly tells you to buy the maker's own filters or sprays. Watch the five numbers above, learn their targets, and you have covered the vast majority of what affects your health at home.

What professionals know

These five metrics were chosen because each has a well-established health or comfort basis and can be measured affordably. PM2.5 aligns with WHO and EPA guidance; the WHO annual guideline sits at 5 µg/m³, which is why that is the "good" threshold. CO2 is a proxy, not a toxin at these levels: it correlates with occupancy and poor ventilation, and studies on cognition (notably Harvard's COGfx work) show measurable declines in decision-making performance as CO2 climbs past roughly 1000 ppm.

The important nuance is measurement quality per metric. CO2 sensors that use true NDIR (nondispersive infrared) technology are genuinely accurate and worth insisting on. Humidity and temperature sensors are cheap and reliable across almost all devices. The weak links are consumer VOC and, to a lesser extent, PM sensors. Most low-cost VOC sensors are metal-oxide (MOX) types that report a relative index rather than a calibrated concentration, so a "250 ppb" reading from a $100 monitor should be treated as directional, not absolute. Use VOC readings to spot changes (a spike after painting), not to certify a specific chemical level.

Cross-checking matters. If your monitor claims dangerous PM2.5 but the air looks and smells clean and no source is present, suspect the sensor before panicking. Reference-grade data is available for outdoor context from EPA AirNow and community PurpleAir networks, which can help you sanity-check your indoor device against local conditions. Sources for targets: US EPA, World Health Organization air quality guidelines, ASHRAE ventilation and comfort standards.

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