## 3 Results  
The discharge of sunscreen in aquatic systems is dependent on several factors, including the total number of tourists in a region, and regulations imposed by tourism sites and by environmental legislation governing the usage of sunscreens. The estimated total discharge of sunscreen (TDS) in Quintana Roo is high. In 2019, we estimate that between 231 and 313 tons of sunscreen were discharged into fresh and marine aquatic ecosystems. We found that the largest quantities of sunscreen were discharged at beaches, followed by sinkholes, and finally inland aquatic parks (Figure 1). Figure 1: Estimated total sunscreen discharge by Casas-Beltran [1] and the present study for the last 12 years of tourist activity in the state of Quintana Roo, México. To determine total calculated tons (TCT), we used field-generated survey data to adjust the total estimated tons. TCT is broken down by study area: aquatic parks, sinkholes, and beaches. In addition, our analysis of estimated discharge over the last 12 years demonstrated an increase in sunscreen discharge at beaches, water parks, and cenotes (Figure 1). The largest discharges of sunscreen occurred in the tourist destinations of Cancun and the Riviera Maya, where we estimated that more than 500 tons of sunscreen were discharged in the last 12 years. Isla Mujeres and Chetumal are the municipalities with the least total estimated discharge of sunscreens. However, the rate of increase over the last 12 years varied by municipality. The estimated total sunscreen discharge at Isla Mujeres quintupled, from 2 tons in 2007 to more than 10 tons in 2019. Cancun and the Riviera Maya doubled their estimated total discharge over the past 12 years. Chetumal, as a tourist destination, increased from a discharge of five tons in 2007 to less than nine tons in 2019. Annual sunscreen discharge in Cozumel doubled over the last 12 years (Figure 2). Based on the results of our survey of visitors to the region, we found that approximately 29% of visitors apply sunscreen when visiting a beach, and they apply it on average 4.5 times per day. Sinkholes and water parks often require visitors to not apply sunscreen, which is reflected in our findings that only 2% of visitors to water parks applied sunscreen and they applied it an average of 1.5 times during their visit. Similarly, only 4% of visitors reported that they apply sunscreen when they visit a sinkhole, again applying an average of 1.5 times a day. Understanding what this means in terms of total sunscreen discharged to the region is challenging because the exact number of visitors per year to different locations is unknown as there are no public data available on total visitors. The discharge calculated in the present work was adjusted with the TBS, TVEC, and TAQ values, which depend on the behavior of the tourist. This behavior can be modified by environmental legislation. Our results of the analysis of environmental legislation are described below. There are currently no regulations regarding the use of sunscreen at beaches in the Riviera Maya, and only limited regulations elsewhere. Additionally, visitors differ in their understanding of the potential environmental harm caused by different types of sunscreen. Figure 2: Estimation of the total discharge of sunscreen (TDS) in the five main tourist destinations of Quintana Roo, México, over the last 12 years. (**A**) Total discharge of sunscreen in four important reef areas of Quintana Roo, México. (**B**) Total discharge of sunscreen from 2007 to 2019 in the five principal tourism states of Quintana Roo. From our survey of visitors to the region, we found that 78% of people understood that conventional sunscreen can cause pollution, and 58% said biodegradable sunscreen can also be a pollutant. Despite this, the vast majority chose to use conventional sunscreens (not biodegradable), while 10% mentioned using biodegradable or ecological sunscreens.