Welcome back to Air Quality Matters and One Take, where we unpack the latest research shaping our understanding of indoor air and the built environment.

This week, we're at a paper that tackles a fundamental question: Where exactly should we put indoor air quality sensors? How many do we actually need? And how often should they take readings?

The paper is titled Long-term Indoor Air Quality Monitoring in Office Buildings: Data-Driven and Goal-Oriented Recommendations for Sensor Replacement and Sampling Frequency, and it's all about moving from guesswork to evidence-based monitoring strategies.

The Problem:

Existing standards are all over the place. One might say you need a sensor every 50 square meters, another says 325 is fine. One says sample every 15 minutes, another says 30 is okay. There's a real lack of clear, evidence-based guidance—and if you get it wrong, you're either spending a fortune on equipment you don't need, or you're collecting data that's useless or actively misleading.

The Study:

Researchers installed 16 sensors across three office sites in Shanghai and collected four years of data on PM2.5, PM10, and CO₂—taking readings every single minute. They then analyzed the spatial and temporal patterns to answer the critical questions: Does a sensor over here tell the same story as one over there? And what information do you lose if you sample every hour instead of every minute?

Key Findings:

Particulate Matter (PM2.5 & PM10): The dominant driver is outdoor air. Indoor PM levels tracked outdoor levels very closely, meaning infiltration is the key factor. For tracking general trends, you don't need a massive density of sensors—readings from different locations were highly correlated.

CO₂: A Completely Different Story: CO₂ is generated indoors by occupants, and the study found huge differences depending on sensor location. Key factors included the type of HVAC system, room size, distance from windows, and proximity to air outlets. A single sensor in a large, complex office space just won't cut it.

The Goal-Oriented Approach:

Here's where the paper gets clever. The authors argue that we're often trying to do two different jobs with monitoring, and we need to separate them:

Job #1 - Temporal Trend Monitoring: Understanding the big picture—daily cycles, seasonal changes, overall average conditions. For this, you can use about one sensor every 150 square meters, and sampling intervals of 90 minutes for PM and 130 minutes for CO₂ are sufficient without losing accuracy. This means less data to store, less energy use, and less maintenance.

Job #2 - High Concentration Event Monitoring: Catching the bad stuff—short, sharp spikes in pollution when a meeting room fills up or outdoor smoke floods in. For this, you need much more frequent sampling: every 4 minutes for PM2.5 and CO₂, and every 15 minutes for PM10. You also need more sensors placed strategically in high-risk zones—large rooms, spaces with standalone AC units, and areas far from ventilation sources.

The Big Takeaway:

There's no single "right" way to monitor indoor air quality—it all depends on your goal. This paper gives us a data-driven framework for making that choice. If you want to understand long-term building performance, you can use a sparse network sampling infrequently. But if your primary goal is health and safety—protecting occupants from pollution peaks—you need a denser, targeted network sampling much more often.

This is about being deliberate. It's about monitoring with purpose.

Long-term indoor air quality monitoring in office buildings: Data-driven and goal-oriented recommendations for sensor placement and sampling frequency

https://doi.org/10.1016/j.buildenv.2025.113392

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Chapters

• 00:00:00 Introduction: The Fundamental Questions of Air Quality Monitoring
• 00:01:35 The Problem: Inconsistent Standards and Guidance Gaps
• 00:02:02 The Study Design: Four Years of Data from Shanghai Offices
• 00:02:38 Particulate Matter Findings: Outdoor Air Drives Indoor Trends
• 00:03:14 CO2 Findings: The Complexity of Indoor-Generated Pollutants
• 00:03:47 The Goal-Oriented Approach: Two Different Monitoring Jobs
• 00:04:37 Trend Monitoring: Less is More for General Performance
• 00:05:19 Event Monitoring: Frequent Sampling for Health and Safety
• 00:06:15 The Big Takeaway: No Single Right Way to Monitor
• 00:06:59 Closing: From Good Ideas to Smart Strategies