What sensors actually measure
Most multi-parameter monitors sold for residential use track four or five indicators: carbon dioxide (CO₂), fine particulate matter (PM2.5), total volatile organic compounds (TVOC), temperature and relative humidity. Some add formaldehyde (HCHO) or radon as separate channels.
Each pollutant requires a different detection principle. CO₂ is detected reliably only by NDIR (non-dispersive infrared) sensors — the same technology used in laboratory instruments. Devices that use electrochemical proxies or estimate CO₂ from VOC readings (often labelled "equivalent CO₂" or eCO₂) are not measuring the gas directly and produce readings that drift significantly in typical apartment conditions.
CO₂ — the most actionable indicator
Carbon dioxide is exhaled with every breath. In an occupied room without mechanical ventilation, concentrations climb steadily. The World Health Organization and European standard EN 13779 class air quality by CO₂ level: below 800 ppm is considered good; 800–1,200 ppm is acceptable; above 1,500 ppm air quality is poor. At 2,500 ppm — easily reached in a sealed bedroom overnight — cognitive test scores in studies drop measurably.
Czech apartments built before 2000 typically rely on window gaps and bathroom extract ducts for ventilation. When residents install new double-glazed windows without compensating airflow, overnight CO₂ can peak above 3,000 ppm. A CO₂ sensor with an audible or visual alert is the most direct tool to identify when to open a window or when mechanical ventilation is needed.
PM2.5 — what arrives from outside (and from inside)
Fine particles smaller than 2.5 micrometres pass through the upper respiratory tract and reach the lungs. Czech cities sit in a geographical basin that traps pollution during winter inversions; ČHMÚ annual reports document PM2.5 exceedances across Prague, Brno and the Ostrava industrial region.
Indoor sources include cooking smoke, candles, incense and certain cleaning products. Laser-particle-counter sensors (used in most consumer monitors) count particles of specific sizes and convert the count to a mass concentration estimate. Readings from different devices are not directly comparable because conversion factors vary — but relative trends within a single device are reliable enough for practical use.
TVOC — a rough signal, not a diagnostic tool
Metal oxide semiconductor (MOS) sensors respond to a broad class of organic gases — everything from acetone nail-varnish remover to off-gassing from new furniture. The TVOC number is a proxy for "something is in the air" rather than a precise measurement of a specific compound. It is useful for identifying unusual events: a freshly painted room, a gas cooker without extraction, or a cleaning product used in an unventilated space.
Sensor technologies compared
Understanding the detection method helps set realistic expectations for accuracy and drift over time.
- NDIR (CO₂): Gold standard for CO₂. Accurate to ±50–75 ppm when factory-calibrated. Most quality NDIR sensors self-calibrate against ambient outdoor baseline (400 ppm). Reliable lifespan of 10+ years.
- Laser particle counter (PM2.5): Counts particles in a light-scattering chamber. Susceptible to high humidity environments — at relative humidity above 80 %, particles absorb moisture and appear larger, inflating readings. Most sensors apply a correction factor; quality varies.
- MOS (TVOC): Fast response time, low cost. Sensitive to humidity and temperature changes, which means readings shift even without a real change in VOC concentration. Requires burn-in time after purchase (24–72 hours).
- Electrochemical (formaldehyde, CO): More specific than MOS for certain gases. Sensor electrode degrades over 2–5 years, requiring replacement. Exposure to competing gases can cause cross-sensitivity errors.
Practical considerations for Czech apartments
Czech residential buildings present a specific environment. Panel-block construction from the 1970s–1990s often has concrete walls with moderate thermal mass, bathroom extract ducts that draw air passively, and limited airtightness. More recently renovated flats with full window replacement and wall insulation can be significantly more airtight — which amplifies the effect of indoor pollution sources.
Placement matters
CO₂ and TVOC sensors should be placed at head height in the room where you spend the most time — typically a bedroom or living room. Avoid placing them directly above or adjacent to a cooker, kettle or exhaust duct. For PM2.5 monitoring, position the device away from direct air currents from open windows, which will depress readings temporarily.
Connectivity and logging
Devices with cloud logging or local export allow you to review historical trends. This is useful for identifying patterns — for example, confirming that PM2.5 spikes every morning during cooking, or that CO₂ peaks between 3–6 AM in a bedroom. Continuous logging makes it possible to evaluate whether a change (adding an air purifier, adjusting ventilation) had a measurable effect.
Calibration and drift
Consumer sensors drift over time. NDIR CO₂ sensors with automatic baseline calibration (ABC) compensate by assuming the lowest reading over a rolling period represents outdoor air (around 400–420 ppm). This works well if the space is regularly ventilated; it fails if the space is sealed for extended periods. Particle sensors may require replacement after 3–5 years.
Devices documented in Central European conditions
Several devices have user-reported data from Czech and Slovak environments. The Aranet4 (NDIR CO₂ + temperature + humidity + pressure) is widely cited in air quality discussion groups for accuracy and battery longevity. The Netatmo Indoor Module (CO₂ + temperature + humidity + noise) has an established presence in Prague households, supported by the local Netatmo developer API which allows data export to third-party systems. Cheaper Wi-Fi-connected sensors — often sold under various brand names at electronics retailers — typically use MOS sensors for CO₂ estimation and should not be relied on for CO₂ figures.
What a sensor does not replace
Monitoring identifies a problem; it does not resolve it. A persistent CO₂ reading above 1,200 ppm in a living space points to insufficient air exchange — the appropriate response is mechanical ventilation or a disciplined window-opening schedule, not purchasing a more expensive sensor. Similarly, elevated PM2.5 readings call for source control (extraction above cookers, avoiding candles or incense indoors) and filtration — not simply recording the data.
Radon — a naturally occurring radioactive gas that seeps from the ground — is a separate category. Czech geological conditions place large areas of Bohemia in elevated-radon zones, as documented by the State Office for Nuclear Safety (SÚJB). Standard IAQ monitors do not measure radon; a dedicated radon detector (e-PERM, AlphaGuard or similar) is the appropriate instrument.
