What Is Particulate Matter? A Guide to Indoor Air Quality and PM Exposure

Updated May 2026

Most people spend over 90% of their time indoors. That air carries the same mix of contaminants found outside, often at higher concentrations, with no wind or weather to disperse it. Particulate matter, or PM, is the most studied and most consequential category of indoor air pollutants, measurable in fractions of a human hair, capable of reaching the lungs and, at fine enough sizes, the bloodstream.

Since 2008, GPS Air has developed and refined NPBI technology to address PM in commercial HVAC systems.

AIR QUALITY: The Basics

What is air pollution?

Air pollution is the presence of harmful solids, liquids, or gases in concentrations that exceed safe or recommended levels. Indoors or outdoors, the sources and effects differ, but the chemistry does not.

The Clean Air Act of 1970 identifies six criteria air pollutants, each regulated by the EPA:

• Particulate matter: solid and liquid droplets suspended in air, originating from direct sources such as construction sites or unpaved roads, or forming through chemical reactions like sulfur dioxide emissions from power plants. Indoor air also contains biological particulates in this category: viruses, bacteria, pollen, and dander.

• Ground-level ozone (O3): forms when nitrogen oxides and volatile organic compounds react in sunlight. Protective in the upper atmosphere, harmful at ground level.

• Carbon monoxide (CO): odorless and colorless, produced by burning gasoline, wood, or propane. Accumulates when ventilation is inadequate.

• Sulfur dioxide (SO2): generated primarily by diesel engines and industrial boilers.

• Nitrogen dioxide (NO2): produced from combustion in cars, trucks, power plants, and off-road equipment.

• Lead (Pb): a naturally occurring element also emitted by some industrial processes.

How does air pollution impact the environment?

Airborne contaminants alter ecosystems in measurable ways. Nitrogen oxides and sulfur compounds create acid rain, which acidifies water and soil and damages vegetation. Fine particle dispersion reduces atmospheric transparency, producing haze. Black carbon, a byproduct of combustion, absorbs solar radiation and accelerates glacial and sea ice melt.

Health effects of air pollution

WHO data shows that 99% of the global population breathes air that exceeds WHO guideline limits, contributing to more than 7 million premature deaths annually.

Health effects range from temporary irritation, eye, nose, and throat discomfort, to long-term cardiovascular and pulmonary disease. Vulnerable populations face disproportionate risk:

• Children, infants, and adolescents
• People with asthma, COPD, or other lung conditions
• People with cardiovascular disease
• Current or former smokers
• People in low-income households or high-pollution areas

How air quality is measured: the AQI

The EPA's Air Quality Index provides a single daily number for each of the six regulated pollutants, drawn from thousands of monitoring stations across the country. The scale runs from 0 to 500 across six color-coded categories:

Real-time AQI readings are available at AirNow.gov and through most state and local agency websites.

Particulate matter: what it is and why it matters

PM size classifications

Particulate matter is sorted by diameter, measured in micrometers (µm). A human hair is approximately 70 µm across. Most people cannot see individual particles smaller than 10 µm without magnification.

According to ASHRAE, 99.9% of particles in a typical indoor atmosphere are smaller than PM1.0, which is the category hardest to capture and most likely to cause harm.

Where PM comes from

Some particles are emitted directly: dust from construction activity, road debris, combustion byproducts. Others form through chemical reactions in the air itself, soot from automobile exhaust, or particles produced when volatile organic compounds react with ozone.

Health effects of PM exposure

The EPA's research documents effects across particle sizes, with smaller particles carrying greater risk. PM10 particles settle in the nose and throat. PM2.5 particles reach deep into the lungs. PM1.0 particles can enter the bloodstream.

Who is most at risk?

Children, older adults, and people with coronary artery disease, congestive heart failure, asthma, or COPD face the highest exposure risk. Health impacts include irritation of the eyes, nose, throat, and lungs; aggravation of respiratory and cardiovascular symptoms; and, with prolonged exposure, premature death in people with pre-existing heart or lung disease.

How to reduce indoor PM levels

Three-layer approach

Source control and ventilation are the starting points: reducing what enters the space and diluting what is already there. Neither passive approach addresses the fraction of particles that circulate continuously without settling. Active air cleaning is the third layer, the only one that works on what ventilation leaves behind.

The MERV filter gap

Filters have a documented blind spot. Research spanning at least 20 years shows they perform counterintuitively in the ultrafine range, specifically between 0.10 µm and 0.40 µm. Filters capture particles below 0.01 µm and above 0.40 µm more efficiently than they capture the particles in between. Very small particles behave like gas molecules and are captured by diffusion; larger ones are stopped mechanically. Mid-range particles fall between both mechanisms.

This is a structural limitation, not a function of MERV rating. A higher MERV rating narrows the gap. It does not close it.

How NPBI addresses the gap

GPS Air's NPBI (needlepoint bipolar ionization) technology targets this filtration gap directly. NPBI devices are installed in HVAC ductwork. As air passes over the ionizer, positive and negative ions are delivered into the occupied space. Those ions attach to airborne particles, causing them to cluster into larger aggregates, a process called agglomeration, that HVAC filters capture more effectively.

Third-party testing

GPS Air's NPBI technology has been independently tested and validated for airborne particle reduction. UL 2998 certified, zero ozone emissions.

See the science behind NPBI particle reduction

The state of U.S. air quality

The EPA reports that combined emissions of the six criteria pollutants fell by 78% between 1970 and 2020, driven by the Clean Air Act and successive rounds of stationary and mobile source regulation.

Outdoor air quality has improved steadily. Indoor air quality has not kept pace. Buildings are sealed tighter for energy efficiency, which concentrates pollutants generated inside. The outdoor trend does not transfer automatically indoors, which is why addressing PM at the source, in the duct, before it recirculates, remains the more direct intervention.

Learn how NPBI targets PM2.5 and ultrafine particles in commercial buildings