**7. Conclusions**

While there are complex issues at play in the distribution and effects of malodors (e.g., pollution concentrated near low-income communities, lack of access to proper sanitation), malodors are a general fact of daily human life. Indoor malodors are particularly challenging for people in developing countries or in low-income communities [2], which may lack the financial resources or opportunities to directly change the sources or living situations that harbor malodors. In these scenarios, malodors ultimately contribute to broad issues of structural inequality [41,98].

Viewing malodors as a merely "aesthetic" issue ignores their potential for negative impact on human health, previously defined as a "state of complete physical, mental and social well-being and not merely the absence of disease or infirmity" [3]. Review of the current literature includes several studies from a diverse range of disciplines reporting negative psychological, physical, social and economic consequences of indoor malodor. Conversely, removal of malodor has been reported to increase performance and subjective responses in workers [10], highlighting the potential benefits of mitigating malodors in indoor spaces. However, there are several gaps in the current research. Specifically, there is a lack of understanding regarding the mechanisms by which malodors can elicit any adverse effects, whether through an individual's experience or expectations that provide the interpretative context in which a VOC is experienced or through a more direct effect on human physiology and well-being. Thus, well-controlled studies examining the emotional, behavioral and performance-related outcomes induced by exposure to malodors are needed, as are studies that include a formal examination of the individual variables (i.e., personality, gender, age, culture) that may influence the magnitude and direction of malodor effects.

Eliminating the source of malodor can be a direct mode of intervening in odorous indoor environments, though it is often not achievable with the resources at hand, particularly in low-income communities. Therefore, easily accessible and affordable approaches to eliminate malodors such as air fresheners with odor eliminating technologies (Table 2) may hold promise for reducing some of the negative effects of indoor malodor. However, we found relatively few investigations into the effectiveness of such measures on improving health outcomes such as cognition, mood, and stress levels, among others. Therefore, further study is recommended on the impact of air fresheners and other odor mitigating products on health outcomes via malodor elimination and/or emission of pleasant fragrances, as well as their impact on measures of overall indoor air quality.

**Author Contributions:** The authors (P.D., A.-S.C., S.H.) contributed equally to the work by providing substantial contributions to the conception and design of the work and the acquisition, analysis, and interpretation of data; drafting the work and revising it critically for important intellectual content; and agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All authors have read and agreed to the published version of the manuscript.

**Funding:** The Household and Commercial Products Association (HCPA) provided funding for the preparation of this manuscript.

**Acknowledgments:** The authors would like to thank Eric Moorhead of Spectrum Science Communications and Mary Begovic Johnson of The Procter & Gamble Company for their helpful reviews and comments during the writing of this manuscript.

**Conflicts of Interest:** Pamela Dalton is a consultant/grantee or speaker for the following companies: American Chemistry Council, Ajinomoto Co., Inc., Altria Group, Campbell Soup Company, Church & Dwight, The Coca-Cola Company, Diageo, plc, Diana Ingredients, Estee Lauder Inc., Firmenich Incorporated, Fragrance Creators Association, Givaudan SA, GlaxoSmithKline, Intelligent Sensor Technology, Inc., Japan Tobacco Inc., Johnson & Johnson Consumer Products, Kao Corporation, Kellogg, Kerry, Mars, McCormick & Company, Inc., Mead Johnson Nutritionals, Mondel ez International, PepsiCo, Inc., Pfizer, Inc., Procter & Gamble, Reckitt Benckiser Group, Roquette, Royal DSM, Sensonics International, Suntory Holdings Ltd., Symrise, Takasago International Corporation, Tate & Lyle, Unilever Research & Development, Wm. Wrigley Jr. Company, Young Living Essential Oils and Zensho Holdings Co. Ltd. Dr. Dalton received an honorarium from the HCPA for the preparation of this manuscript. Anna-Sara Claeson has no conflicts of interest. Dr. Claeson received an honorarium from the HCPA for the preparation of this manuscript. Steve Horenziak is an employee of The Procter & Gamble Company. The funder (HCPA) had a role in the design of the review, the compilation of published work on the subject and the decision to publish the results. The funder had no role in the analyses or interpretation of the reported data. The authors had full editorial control over the content.
