There are thousands of individual organic compounds in the indoor environment, originating from building and decoration materials, furnishings, occupants, and also from chemical interactions. It is also possible for VOCs to ingress from outdoors through ventilation systems. Consequently, VOC concentrations are often much higher indoors than outdoors (Beko et al., 2020). Although most VOCs are not directly harmful to health, they can produce potentially harmful secondary pollutants such as PM and formaldehyde when oxidised. It is therefore important that we understand the source of indoor VOCs (whether originally from indoors or outdoors) and over a range of typical indoor conditions, so we can design appropriate mitigation measures to improve indoor air quality. Such an understanding will help us to identify when indoor or outdoor mitigation measures are more beneficial. 

Information about specific VOCs related to various indoor sources can be found in the report of Working Group 1, such as from building materials (Chapter 2a); occupants, their activities and household products (Chapters 2b and 2c); and microbial activities (Chapter 2d). Once emitted, the fate of both indoor and outdoor VOCs is controlled to some extent by oxidation, mainly through reaction with the hydroxyl radical (OH) and/or ozone (O3). Indoor concentrations of these oxidants are typically lower than outdoors, owing to increased deposition rates for ozone and much lower light levels to photochemically produce the hydroxyl radicals indoors. 

However, there are other factors that control indoor VOC concentrations, such as ventilation rates with outdoors, internal and external climate, and surface interactions. Higher ventilation rates permit more outdoor VOCs to enter a building, and more internally emitted VOCs to leave. However, such conditions also allow for greater ingress of ozone, leading to higher rates of ozonolysis reactions indoors, which tend to transform double-bonded VOCs such as terpenes to a wide range of oxidised VOCs and particulate matter (e.g. WG1 report). External meteorological conditions can also determine whether outdoor-generated VOCs accumulate outdoors and potentially have a greater chance of entering a building, or are rapidly dispersed such as through high wind speeds. Finally, occupants and their activities can also impact the VOC concentrations indoors, as reviewed in the WG1 report. Therefore, without understanding the specifics of a building, the VOC sources in and around it, and the way the building is used by the occupants, it is impossible to predict whether indoor or outdoor VOC sources dominate the indoor air. 

Ideally, a range of VOCs would be measured simultaneously indoors and outdoors and in real-time. There would also need to be an accurate measure of ventilation rates, as well as the indoor concentrations of the relevant oxidants such as ozone and the hydroxyl radical. Another approach would be to measure the OH reactivity. This measurement provides the overall loss rate of OH. By comparing the measured value with what you expect given the measured VOCs (and given their oxidation reaction rates), you can estimate whether there are ‘missing’ VOCs that have not been determined through your experimental method.  

With current technologies, simultaneous indoor and outdoor measurements of VOCs in real-time would be challenging. Although there exist technologies such as SIFT-MS (selective ion flow tube mass spectrometry) that can measure in real time (Lehnert et al., 2020), such measurements rely on the availability of calibration standards, and understanding the impacts on changes in humidity and temperature typically experienced indoors (e.g. during cooking). SIFT-MS is also unable to distinguish between compounds with the same molecular weight, so can only measure total monoterpene concentration for instance.

The VOC concentrations can also be measured off-line using active or passive sampling techniques, or on-line using sensors. Off-line measurements tend to collect samples on adsorption tubes or in canisters for later analysis by gas chromatography. Although this method would only give a snapshot of the VOC concentrations, it would provide better speciation for a wider range of species when compared to e.g. SIFT-MS. 

Specific measurement recommendations are as follows:

ozone : WG4 work described in Part 5.2.1, p13 in Sampling and analysis techniques for inorganic air pollutants in indoor air

HOx/ROx : WG4 work described in Part 2.1.1-2.1.3, p6 for OH, 2.1.6 p10 for HO2,  p7 for OH reactivity in Techniques for measuring indoor radicals and radical precursors. 

VOCs: refer to Mapping organic constituents and WG4 Full article: An overview of methodologies for the determination of volatile organic compounds in indoor air. Information regarding the possibilities, features and application of sensors for (among others) organic compounds is available in the WG4 sub-report ‘Measurement techniques for indoor pollutants using low-cost sensors’ or in Ródenas[SL4]  et al. (2022).

Finally, one could consider the use of novel interpretative methods (e.g., data mining and/or machine learning algorithms, Principle Component Analysis, Positive Matrix Factorisation) for simplification, reduction and interpretation of the complex chemical data generated through these measurements. Such methodologies can be applied to understand the relative contributions of sources beyond the historical simplifications by use of surrogates, representative compounds and tracers/markers for individual processes, towards a more complete appreciation of their complexity. 

Ideally in a range of building types, though some of these instruments are challenging to use indoors, with demanding power, space, ventilation and noise considerations. Ideally, all of the instruments would be co-located as the radicals have a very short lifetimes and there may be concentration gradients across indoors spaces (especially near windows). 

The related building and ancillary parameters must be measured to understand the their influence on the measured concentrations of the chemical species (WG5 : List of parameters to be measured in ALL buildings, D11). Again, these ideally need to be measured adjacent to, and simultaneously with, the chemical species, as some oxidants are formed by photolysis processes and photolysis fluxes are useful to quantify the production of oxidants. It is also critical that concentrations are measured outside the building, so the relative importance of indoor and outdoor sources and environmental parameters on indoor concentrations can be determined.