What is the nature of organic compounds in the indoor environment?

There are thousands of individual organic compounds in the indoor environment, originating from indoor building and decoration materials, furnishings, the occupants, and from outdoors via ventilation systems. The concentrations of some of the indoor organic compounds may be sufficient to pose health hazards. Many of the organic air pollutants are odorous, which is not directly a health risk, but the occupants can be adversely affected by the smell, and odour discomfort can cause problems such as reduced cognitive ability. Given there are numerous organic compounds indoors, we now need to understand the role of individual compounds, so we can better understand sources, health effects and suggest mitigation measures. 

Most previous studies have measured volatile organic compounds (VOCs) indoors as a group, Total VOCs, or TVOCs. The TVOC concept was recently revisited (Salthammer, 2022) and it was concluded that TVOCs is not a toxicologically-based parameter and is limited to screening purposes only. Other studies have focused on a few individual VOCs selected using a risk-based ranking analysis (e.g., Szabados et al, 2021). Measurements of individual compounds are required to be able to elucidate sources, health effects, etc. Information about specific VOCs related to various sources can be found in the report of Working Group 1: building materials (Chapter 2a); occupants, their activities and household products (Chapters 2b and 2c);  microbial activities (Chapter 2d) and source apportionment in Chapter 2f.

A better classification of the organic compounds indoors would be better based on their volatility, rather than identification of myriad individual VOCs. Many of the products used indoors will emit organic compounds that partition between the gas- and particle-phases. For example, fragrances are applied in the condensed phase, and evaporate towards their equilibrium concentration in the vapour phase. The extent to which the vapours contribute to indoor air quality, to the available reactivity or resulting health effects, will depend upon the degree of disequilibrium after the time of application. Guidance on the characterisation of organic compounds according to volatility can be found in the report of Working Group 2, Chapter 1. 

The key species that should be measured will depend on the specific objective of the investigation. More information on the pollutants that might be expected for different sources can be found in the report of Working Group 3. The various sources include emissions from materials and activities, including innovative materials and material ageing (Chapters 3, 4 and 5) and biological activities (Chapter 6). Organic compounds specifically related to occupants (humans) are characterised by Bekö et al. (2022).

The organic compounds to be measured indoors can also be selected and ranked according to their reactivity with the key atmospheric oxidants, hydroxyl and nitrate radicals, and chlorine atoms. Such ranking can also be carried out according to known health impacts. The ranking strategies and list of prioritised compounds are presented in Chapter 7 (reactivity) and Chapter 10 (health impacts). Chapter 8 is dedicated to the identification of the key organic gas species involved in particle formation (particle precursors), and the chemical characterisation of primary particles present indoors (emitted indoors or from outdoors) as a function of their size and composition.

Appropriate techniques should be selected based on the specific objectives of a particular study, the required time resolution and the available funding. The organic compound concentrations can be measured off-line using active or passive sampling techniques, or on-line using direct-reading instruments, or sensors. A practical guide to methods and instrumentation with the specifics on the complexity and competence requirements to perform the sampling and analyses is described in a WG4 sub-report ‘Sampling and instrumentation for organic gas pollutants’ or in Vera et al. (2022). Information regarding the possibilities, features and application of sensors for (among others) organic compounds is available in WG4 sub-report ‘Measurement techniques for indoor pollutants using low-cost sensors’ or in Ródenas[SL4]  et al. (2022). 

Indoor air quality depends on the building characteristics as well as on the occupants and their activities. For a thorough understanding of IAQ in buildings, the air pollutant data must be put into context with the building location, layout, occupancy, ventilation systems, building and furnishing materials, as well as the indoor temperature, relative humidity and air change rates, etc. The final report from Working Group 5 provides a guide to help with selection of the building type and the specific considerations for measurements within each type.