water Article Risk Exposure during Showering and Water-Saving Showers Hélène Niculita-Hirzel 1, * , Sami Goekce 1 , Caroline Eliane Jackson 1,2 , Guillaume Suarez 1 and Luc Amgwerd 2   Citation: Niculita-Hirzel, H.; Goekce, S.; Jackson, C.E.; Suarez, G.; Amgwerd, L. Risk Exposure during Showering and Water-Saving Showers. Water 2021, 13, 2678. https://doi.org/10.3390/w13192678 Academic Editor: Hodon Ryu Received: 31 July 2021 Accepted: 22 September 2021 Published: 28 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department Work, Heath & Environment, Center for Primary Care and Public Health (Unisanté), University of Lausanne, Route de la Corniche 2, 1066 Epalinges, Switzerland; samigoekce@gmail.com (S.G.); caroline@gjosa.ch (C.E.J.); guillaume.suarez@unisante.ch (G.S.) 2 Gjosa SA, La Haute-Route 78, 2502 Biel/Bienne, Switzerland; luc@gjosa.ch * Correspondence: helene.hirzel@unisante.ch Abstract: Eco-friendly showers aim to lower energy and water consumption by generating smaller water droplets than those produced by traditional systems. To evaluate the risk of users inhaling the contaminants associated with such water droplets—namely, chemical components or opportunistic bacterial pathogens such as Legionella—we modeled the behavior of water droplets aerosolized by water-atomization technology at a flow rate of 2.2 L/min and compared the results obtained using this model with those determined experimentally in a typical shower stall. Additionally, we monitored the number and mass of inhalable water droplets emitted by twelve showerheads—eight using water-atomization technology and four using continuous-flow technology—which have distinct characteristics in terms of water flow rate, water pressure, spray angle, and number of and diameter of nozzles. The water-atomizing showers tested not only had lower flow rates, but also larger spray angles, less nozzles, and larger nozzle diameters than those of the continuous-flow showerheads. We observed a difference in the behavior of inhalable water droplets between the two technologies, both unobstructed and with the presence of a mannequin. The evaporation of inhalable water droplets emitted by the water-atomization showers favored a homogenous distribution in the shower stall. In the presence of the mannequin, the number and mass of inhalable droplets increased for the continuous-flow showerheads and decreased for the water-atomization showerheads. The water- atomization showerheads emitted less inhalable water mass than the continuous-flow showerheads did per unit of time; however, they generally emitted a slightly higher number of inhalable droplets (1.6 times more), including those large enough to carry a bacterium each—only one model performed as well as the continuous-flow showerheads in this regard. Further experiments are needed to assess whether this slight increase in the number of inhalable water droplets increases the biological risk. Keywords: aerosols; inhalation exposure; water conservation; flow rate; showerheads; PM 10 1. Introduction Showering represents the largest inhalation exposure to volatile and aerosolized components of water—bioaerosols, metals, and chemical contaminants, e.g., disinfection by-products (DPBs)—in daily life [1,2]. Depending on the nature and concentration of contaminants, both acute and chronic effects of their inhalation have been reported. Long- term daily exposure to high levels of DPBs (e.g., trihalomethanes (THM)) is known to increase the risk of developing cancer [3], and such levels are regularly reported in the litera- ture [4,5]. Consequently, the level of these contaminants in tap water needs to be controlled via regulation, and particular attention must be paid in places where inhalation exposure is significant, e.g., places with low ventilation rates and high shower frequency. Other water contaminants can affect health after short-term exposure alone. Thus, while endotoxins commonly aerosolized in showers are rarely of sufficient concentration to impact human health [6], the inhalation of water aerosols containing opportunistic bacterial pathogens (OBPs)—including, among others, Legionella pneumophila, nontuberculous Mycobacteria, and Pseudomonas aeruginosa—during routine showering has been regularly implicated in Water 2021, 13, 2678. https://doi.org/10.3390/w13192678 https://www.mdpi.com/journal/water