CO₂, PM2.5, and VOCs: What These Numbers Actually Mean for Your Building

If you manage a building — a school, a senior living community, a medical facility, a commercial office — you've probably seen air quality metrics mentioned in the context of health, ventilation, or compliance. CO₂. PM2.5. VOCs. An Air Risk Score of 78.

What do these numbers actually mean? And at what point do they represent a real risk to the people in your building?

CO₂ — The Ventilation Proxy

What it measures: Carbon dioxide concentration in parts per million (ppm).

Carbon dioxide is exhaled by every person in a space. It doesn't cause harm at the concentrations found in buildings, but it serves as the most reliable proxy for ventilation quality. When CO₂ rises, it means fresh outdoor air is not reaching the space at an adequate rate. That means every other airborne hazard — pathogens, VOCs, particulates — is also accumulating.

The thresholds that matter:

| Level | CO₂ (ppm) | What it means | |---|---|---| | Excellent | < 600 | Well-ventilated. Outdoor air is flowing freely. | | Acceptable | 600–800 | Adequate. Within ASHRAE 62.1 guidance. | | Elevated | 800–1,000 | Ventilation is marginal. Risk of pathogen accumulation. | | High | 1,000–1,400 | Studies show measurable decline in cognitive performance. | | Hazardous | > 1,400 | Headaches, fatigue, and decision-making impairment. |

A landmark Harvard study (Environ. Health Perspectives, 2015) found that doubling CO₂ concentrations from 550 to 1,000 ppm reduced cognitive performance scores by 15%. At 2,500 ppm, scores dropped 50%. In a school classroom or office, elevated CO₂ isn't just an HVAC problem — it's a productivity and learning problem.

ASHRAE 62.1 — the ventilation standard referenced by most building codes — recommends maintaining CO₂ at or below 1,100 ppm in occupied spaces. Many high-performance buildings now target 800 ppm or below.

PM2.5 — The Invisible Respiratory Threat

What it measures: Fine particulate matter with a diameter of 2.5 micrometers or smaller, measured in micrograms per cubic meter (µg/m³).

PM2.5 particles are small enough to bypass the nose and throat entirely and deposit directly into the lower lungs. Long-term exposure is linked to respiratory disease, cardiovascular disease, and premature mortality. Short-term spikes — from wildfire smoke, outdoor traffic infiltrating a poorly sealed building, or indoor sources like cooking and cleaning products — can trigger immediate symptoms in sensitive populations.

EPA and WHO thresholds:

| Level | PM2.5 (µg/m³) | Health Implication | |---|---|---| | Good | 0–12 | No risk for healthy individuals. | | Moderate | 12–35 | Sensitive groups may experience symptoms. | | Unhealthy for Sensitive Groups | 35–55 | Children, elderly, and those with respiratory conditions at risk. | | Unhealthy | 55–150 | Everyone may begin to experience effects. | | Hazardous | > 150 | Serious health effects for all occupants. |

For schools, senior living facilities, and healthcare environments — where the occupants are disproportionately vulnerable — the threshold for concern is lower than it is for a healthy working-age adult. A PM2.5 reading of 35 µg/m³ in a senior living common area carries a very different risk profile than the same reading in a commercial office.

VOCs — The Silent Chemical Load

What it measures: Volatile Organic Compounds — a broad class of carbon-based chemicals that evaporate at room temperature, expressed qualitatively (Good / Moderate / Poor) or in ppb (parts per billion) when measured by sensor.

VOCs come from dozens of indoor sources: paint, adhesives, furniture off-gassing, cleaning products, personal care products, photocopiers, and building materials. Many are benign in trace quantities. Others — formaldehyde, benzene, toluene — are known carcinogens or respiratory irritants at higher concentrations.

What matters most about VOCs:

Cumulative burden: A single VOC source at low concentration may be harmless. Dozens of sources in a sealed, poorly ventilated space create a chemical cocktail that is harder to evaluate and easier to underestimate.
Occupant sensitivity: Children, elderly occupants, and immunocompromised individuals are more sensitive to VOC exposure than healthy adults.
Trend data is more valuable than point readings: A single VOC reading means little. Trending data shows you when a building is accumulating chemical load — which could indicate a new contamination source, a ventilation failure, or an unusual cleaning event.

The Air Risk Score — Combining Them Into One Number

The problem with monitoring CO₂, PM2.5, VOCs, temperature, and humidity separately is that a building can score well on two metrics while failing dangerously on a third. A composite score removes that blind spot.

The Air Risk Score (0–100) combines all sensor readings into a single, instantly interpretable index:

0–40 (Green): Air quality is healthy. No action needed.
41–70 (Amber): Moderate risk. Ventilation adjustment or investigation recommended.
71–100 (Red): Elevated risk. Immediate action required.

A score of 78 — as you might see in an under-ventilated wing of a school building in winter — signals that CO₂ and VOCs are elevated enough that occupant health and performance are being affected, even if no one has complained yet.

Why Continuous Monitoring Changes Everything

Periodic air quality testing — an annual inspection, a one-time sensor deployment — tells you what the air was like on one specific day under one set of conditions. It tells you nothing about what happens at 2 PM on a Friday when a classroom is fully occupied, HVAC maintenance is overdue, and it's -5°F outside.

Continuous, real-time monitoring answers the question that matters: What is the air quality right now, in every space, and is it trending toward a problem?

That is the difference between managing air quality reactively — waiting for complaints, responding to incidents — and managing it proactively, with the data to act before anyone gets sick, before absenteeism spikes, and before a liability event occurs.