**1. Introduction**

The sense of smell is a fundamental means of navigating the sensory world and orienting ourselves to ecologically and socially appropriate behavior. Evolutionarily, this chemical sense was the original means by which the earliest organisms achieved adaptive regulation of action and can be considered the origin of behavior [1]. For humans today, volatile molecules can travel for long distances and thus can provide important information about people, places, food and things that cannot otherwise be immediately detected by other sensory systems. Beyond its informational content, odor can attract, intrigue, impress and entice, as well as repel, offend, disgust, or evoke pity. Malodors are odors perceived to be unpleasant or offensive and, while not necessarily occurring at the known thresholds for direct adverse events, may elicit negative symptoms via the olfactory system's connections to other cognitive and behavioral systems.

Malodors are sometimes depicted as an inconvenience or annoyance of relatively minor importance to human perception and experience [1]. An understanding of malodors as a merely "aesthetic" issue, however, ignores their potential for negative impact on human health and social relations [2]. Malodors

propagate a variety of psychological, social and economic disturbances, many of which are preventable. As defined at the International Health Conference, "health is a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity" [3]. Although crafted in 1946, this definition of health has remained in use by organizations such as the World Health Organization. Combating the sources and mitigating the impacts of malodors therefore represents an important public health undertaking.

Throughout history, people have used perfumes, incense, herbs and other means at their disposal to rid their indoor environments of the malodors that occur in the course of human life and industry. However, within a social structure where people cannot always remove the sources of malodors or move themselves to avoid odors, or where odors are the result of industries that sustain food and energy supplies, frequent and intense malodors that are left unchecked can contribute to larger challenges. Malodors can directly affect physical health if the malodorous chemical represents an irritant or harmful airborne substance and occurs at a high enough concentration to exceed observable adverse effect levels. Additionally, malodors may act indirectly, as a mediator of mood, performance and health symptoms, effects which are the focus of this review [4]. Negative effects of low-level chemical exposures (e.g., malodor exposure) are further discussed in Section 5. As the World Sanitation Foundation notes, malodors that result from poor sanitation can compound sanitation issues in under-resourced and developing areas [5]. In rural India, malodors resulting from poor sanitation in pit latrines can indirectly result in open defecation and thus spark a variety of new community-wide health hazards, including compounded malodor issues [6–8].

While eliminating the source of indoor malodor can be a direct mode of intervening in odorous environments, it is often not achievable with the resources at hand. Even in today's urbanizing societies, where malodors now concentrate indoors and in private spaces, people, especially those in low-income communities, may not have the resources to remove the sources of malodors or to relocate their residence. In this review, we examine and discuss the current state of understanding on the role of indoor malodors for impacting human behavior, performance and health, including the individual factors that may govern such responses and identify research priorities to address the data gaps where they exist. Malodors have been reported to have a number of negative psychological, physical, social and economic consequences, as will be discussed herein. Conversely, removal of malodor by increased ventilation or filtering has been reported to increase performance and subjective responses in workers (e.g., ratings of air freshness and air quality), highlighting the potential benefits of mitigating malodors in indoor spaces [9,10]. However, such interventions may not be feasible in many indoor environments. Malodor is an important part of indoor air quality, and accessible and affordable malodor solutions such as air fresheners should be studied to determine if similar benefits are observed.
