What Are VOCs?

Indoor VOC concentrations are typically two to five times outdoor concentrations, and an order of magnitude higher during construction or refurbishment. The major sources cluster into:

Building materials. Paints, varnishes, sealants, adhesives, MDF, particleboard, vinyl flooring, foams. Formaldehyde from urea-formaldehyde resins is the canonical long-term emitter.

Furnishings and textiles. Upholstery, carpet underlay, mattresses and curtains release plasticisers and residual solvents for months.

Cleaning and consumer products. Cleaning chemicals, air fresheners, scented candles, personal-care products and toner. Limonene and pinene from "natural" products react with ozone to form formaldehyde and ultrafine particles.

Occupant activity. Cooking, smoking and even exhaled breath. Acetone and isoprene from human metabolism are routine background contributors.

Outdoor infiltration. Traffic-source benzene and toluene, industrial emissions, biogenic terpenes.

TVOC is a useful screening metric. It collapses a mixture of dozens of compounds into a single number, suitable for trend visualisation, ventilation diagnostics and occupant alerts. It is not a health-risk metric.

A TVOC of 800 µg/m³ dominated by limonene from a citrus cleaner is materially different from 800 µg/m³ dominated by formaldehyde or benzene. The health-relevant questions — exposure to carcinogens, respiratory sensitisers, neurotoxic solvents — depend on which compounds are present, not the sum of all of them.

Speciated measurement uses active or passive sampling onto Tenax or activated charcoal sorbent tubes, followed by thermal desorption GC-MS in a laboratory. Detection limits below 1 µg/m³ per compound are routine; the cost is per-tube and the turnaround is several days. The standard workflow is continuous TVOC sensing to identify when and where peaks occur, followed by targeted sorbent sampling during a peak to identify what is in it.

Photoionisation detectors (PID) are the dominant TVOC technology for portable and field instruments. A UV lamp (typically 10.6 eV) ionises VOCs in a sample chamber; the resulting current is proportional to the total ionisable mass. PID is sensitive (sub-ppb for aromatic solvents), fast (sub-second response) and broadly responsive across the VOC range — but blind to compounds with ionisation potentials above the lamp energy. See PID monitors for instrument detail.

Metal-oxide semiconductor (MOS) sensors use a heated semiconductor whose conductivity changes when reducing gases adsorb on its surface. MOS sensors are cheap, low-power and ubiquitous in networked indicative monitors. They are non-linear, humidity-sensitive and prone to drift, so they require periodic field reference.

Electrochemical cells are used for individual reactive compounds — formaldehyde, hydrogen sulphide, ammonia. Compound-specific, two-to-five year lifetime.

Laser spectroscopy (typically tunable diode laser absorption) is emerging for continuous formaldehyde monitoring at part-per-billion sensitivity.

Formaldehyde is the single most common indoor VOC of concern in the UK. WHO classifies it as a Class 1 human carcinogen and sets a 30-minute guideline of 100 µg/m³. Sources are pressed-wood products, urea-formaldehyde insulation, combustion and secondary chemistry from terpene-ozone reactions.

Standard PID sensors miss formaldehyde because its ionisation potential (10.88 eV) exceeds the 10.6 eV lamp energy. Reliable formaldehyde monitoring requires either DNPH-cartridge laboratory sampling, dedicated electrochemical formaldehyde sensors (suitable for indicative continuous monitoring), or laser-based continuous instruments. See formaldehyde monitoring.

A TVOC sensor will tell you that something is emitting. It will not tell you what, how toxic it is, or whether the people in the room are at risk. Escalation triggers are: TVOC persistently above 1,000 µg/m³, occupant symptoms inconsistent with CO₂ or PM, suspicion of a specific compound (formaldehyde after new MDF, benzene near a garage, naphthalene from new flooring), or any clearance test required after refurbishment.

Escalation means speciated sorbent sampling followed by GC-MS, scoped to the compounds plausibly present. Pair the lab results with the continuous TVOC trend to identify when emissions peaked and what controls (ventilation rate, source removal, activated-carbon filtration) reduce them.

• Is TVOC the same as total VOC concentration?

  Not quite. TVOC is a sensor-reported summed response to whichever compounds the detector responds to, weighted by its sensitivity to each. A PID using a 10.6 eV lamp will not see methane, formaldehyde or methanol; an MOS sensor will not respond uniformly across the alkane series. TVOC values from different sensor types are not directly comparable.

• What is a healthy TVOC level indoors?

  Indoor TVOC below 300 µg/m³ is generally considered low; 300–1,000 µg/m³ warrants investigation; above 1,000 µg/m³ usually triggers source identification. These bands come from German Federal Environment Agency guidance and are widely used in UK indoor air practice.

• When is speciated sampling needed instead of a TVOC sensor?

  When TVOC is persistently elevated, when occupants report symptoms that may be substance-specific, when a regulated compound (formaldehyde, benzene, naphthalene) is suspected, or when a clearance test is required after refurbishment. Speciated sampling uses sorbent tubes and laboratory GC-MS.

• Do PID sensors detect formaldehyde?

  Standard PIDs with 10.6 eV lamps do not — formaldehyde's ionisation potential is 10.88 eV. Detection requires either an 11.7 eV PID lamp (short-lived, expensive) or a dedicated formaldehyde sensor using electrochemical or laser-based detection.