What Are PM2.5, HCHO, and TVOC? A Simple Guide to Indoor Air Pollutants

The world of indoor air quality is filled with an alphabet soup of acronyms: PM2.5, HCHO, TVOC, CO2. These terms appear on air purifier boxes, in news reports about wildfires, and on the screens of home air monitors. This can be confusing when all you know is that your home or office feels “stuffy” or “off.”

You do not need a chemistry degree to understand what you are breathing. This guide decodes the most common—and most important—metrics for indoor air.

According to the U.S. Environmental Protection Agency (EPA), the air inside our homes and workplaces can be two to five times more polluted than the air outside. These pollutants generally fall into two distinct families: Particulate Matter (particles) and Chemical Compounds (gases), plus a third category of Environmental Indicators that provide critical context.

Family 1: Particulate Matter (PM) - The “Dusts and Soots”

“PM” stands for Particulate Matter, a term for tiny solid particles and liquid droplets suspended in the air. The number (10, 2.5, or 1.0) refers to their size in micrometers. A smaller number means a more dangerous particle.

• PM10 (The “Irritant”): These are coarse particles under 10 micrometers. Think of pollen, dust mites, and mold spores. They are large enough to be trapped in your nose and upper respiratory tract, where they can trigger allergies, coughing, and sneezing.
• PM2.5 (The “Invader”): This is the metric most commonly cited for air pollution. At 2.5 micrometers, these fine particles are 30 times smaller than a human hair. They are small enough to bypass your body’s natural defenses and travel deep into the lungs, causing respiratory inflammation and aggravating conditions like asthma. Common sources include wildfire smoke, cooking (especially frying), vehicle exhaust, and burning candles.
• PM1.0 (The “Penetrator”): These are ultrafine particles. They are so minuscule that they can pass from the lungs directly into the bloodstream, allowing them to travel to and impact other organs, including the heart and brain.

Family 2: Chemical Gases (VOCs) - The “Fumes and Odors”

This group is different. These are not particles and cannot be caught by a standard particle filter. They are the source of that “new” smell that is often a sign of pollution.

• HCHO (Formaldehyde): This is a specific chemical to watch. Formaldehyde is a colorless, strong-smelling gas used in the manufacturing of countless building materials and household goods. It is a known irritant that can cause watery eyes, coughing, and a burning sensation in the throat.
  • Primary Sources: Pressed-wood products (MDF, particleboard, plywood) used in new furniture, cabinets, and flooring. Also found in glues, adhesives, and some fabrics. That “new furniture smell” is often formaldehyde “off-gassing.”
• TVOC (Total Volatile Organic Compounds): This is not a single chemical but a total measurement of a large group of them. Think of it as a “chemical soup” score. Formaldehyde is one type of VOC, but thousands of others exist.
  • Primary Sources: Paints, varnishes, cleaning supplies, disinfectants, air fresheners, pesticides, and scented candles. The “V” in VOC stands for “volatile,” meaning these chemicals easily evaporate into the air at room temperature, causing headaches, nausea, and that “stuffy” feeling.

Family 3: Environmental Indicators - The “Context”

This group isn’t pollution, but monitoring them is essential for managing the other two families.

• CO2 (Carbon Dioxide): This is the “Master Indicator.” CO2 is what you exhale. At normal levels, it is not toxic, but it is the single best proxy for your home’s ventilation.
  • The “Stuffy Room” Effect: That sleepy, foggy-headed feeling in a crowded meeting is a symptom of high CO2 levels (e.g., over 1000 ppm).
  • Why It Is Critical: A high CO2 reading is a clear sign of poor ventilation. This means you are not getting enough fresh air. More importantly, it means you are also trapping and concentrating all the other pollutants—the PM2.5 from your kitchen and the HCHO from your new bookcase. A high CO2 level is your monitor’s way of saying, “Open a window. You are breathing stale, concentrated air.”
• Temperature & Humidity: High humidity (over 60%) creates a breeding ground for mold and dust mites (which are sources of PM10). Low humidity (under 30%) can dry out your sinuses, making you more susceptible to irritation from dust and chemicals.

Making the Invisible Visible

This “alphabet soup” of invisible metrics is why modern indoor air quality monitors exist. They are not just single-function meters; they are all-in-one dashboards that translate this complex world into simple, actionable numbers.

A multi-function device, such as the AQItech 9AQI+7, serves as a perfect example. It integrates sensors for all the metrics we’ve discussed (PM1.0/2.5/10, HCHO, TVOC, CO2, Temp, and Hum) onto a single, clear display.

This allows you to see, at a glance, the direct relationship between your actions and your air quality: * “My PM2.5 just spiked.” (You just started searing a steak.) * “My HCHO levels are high.” (The new cabinet is still off-gassing.) * “My CO2 is climbing.” (The windows have been closed all day.)

Features like “AQI Alert Buzzers” on such units act as an automatic warning system, doing the monitoring for you and alerting you only when a specific metric requires your attention.

Actionable Solutions: How to Respond to the Data

Now that you can decode the air, here are the primary levers for improving it.

1. Ventilate (The CO2 Fix): This is your most powerful, effective, and cheapest tool. When you see CO2 levels rising, open windows for 10-15 minutes to create a cross-breeze. This purges all pollutants at once.
2. Control the Source (The HCHO & TVOC Fix): The best way to manage chemical pollution is to prevent it from entering. Look for “low-VOC” or “zero-VOC” labels on paint and furniture. Let new furniture “air out” in a well-ventilated space (like a garage) for a few days before bringing it in.
3. Clean and Filter (The PM Fix): Use a vacuum with a HEPA filter to trap fine particles. Use an exhaust fan every time you cook. A high-quality air purifier with a True HEPA filter is designed to capture PM2.5, while one with a thick Activated Carbon filter is necessary to adsorb VOCs and HCHO.
4. Manage Humidity (The Mold & Comfort Fix): Aim for the “Goldilocks” zone of 30-50% humidity. Use a dehumidifier in damp basements or humid summers and a humidifier in dry winters.

You no longer need to guess about your air. You now have the framework to decode these invisible metrics and take targeted, effective action.