**3. Modern Indoor Malodor Challenges Associated with Urbanization**

As malodors in public spaces have generally decreased with post-industrialism sanitation improvements, the domestic household has become a prominent site for exposure to malodors. While malodors experienced in historical periods were concentrated in shared areas, contemporary experiences with malodors are frequently experienced in personal spaces [13]. Contemporary building and insulation techniques used in modern homes can allow malodors to concentrate within the household [14].

Household odors are a combination of external odors that enter the home and odors produced within the home. External odors that invade the home include emissions from industry and pollution. Odors produced within the home arise from aggregate effects of low concentrations of volatile organic compounds (VOC) caused by cooking, pet, and human body odors, and the use of personal and household cleaning products, among others. They can also arise from microbial volatile organic compounds (MVOC) formed by the metabolic processes of fungi and bacteria present on building materials [15]. Over time, these VOC will become absorbed by the porous surfaces in homes such as carpets, soft furnishings, curtains, wall paper and even the grout between tiles. The combination of the bouquet of VOC present in households imparts each home with its own unique smell [16].

Exposure to certain mixtures of MVOC and VOC has been shown to increase reports of poor air quality within indoor spaces [17]. However, it should be noted that VOC as a class of compounds are not inherently toxic or malodorous. With respect to establishing toxicity, one must measure the levels and refer to the known threshold for adverse effect for each specific VOC. Many indoor VOC are perceived to have a pleasant smell and can have positive associations, including the wide variety of VOC that are released during such activities as baking bread or cooking. Additionally, there is a certain amount of subjectivity in an individual's response to specific VOC, as one person may report a positive reaction to a certain VOC based on pleasant memories associated with that VOC while others may report it as a malodor.

The perception of VOC also differs with respect to concentration and context. For instance, the substance skatole (3-methylindole) is present in flowers and essential oils and is frequently used in fine fragrances at low concentrations. However, at higher concentrations, it is perceived as having a distinct fecal odor, pointing to the importance of concentration with respect to malodor perception [18]. The context in which a VOC is interpreted is also critical to how it is perceived. Participants who were told that isovaleric acid (a cheesy-smelling fatty acid) was a body odor rated it as far more unpleasant than participants who were told it was a food odor [19]. Finally, genetic variation across the population has resulted in a 'highly personalized inventory of functional olfactory receptors' that not only determine what any individual can smell but how pleasant or unpleasant an odorant is perceived to be [20–22].

In today's urban societies, people can spend nearly 90% of their time in indoor environments [23,24]. As such, there has been investigation into whether the experience of indoor malodors should be regarded as a health issue or a merely aesthetic one [2,25,26]. This line of inquiry has encompassed field studies of industries that emit high quantities of malodorous compounds in residential areas, surveys of workplace productivity in specific chemical environments, psychological laboratory tests of odor exposure and case studies of heightened olfactory sensitivity, in addition to genetic and neurophysiological studies of olfaction and related biological systems. These types of studies have yielded important insights into understanding of the diverse effects of odors on health and social interactions.
