Children play at the beach in very intimate ways. They tend to sit in the intertidal zone, dig in the sand, and may interact with seaweed that strands on the shore. Given the play activities, children face heightened exposure risks from contaminants in this area, particularly through ingestion, dermal contact, and inhalation (Altomare et al. 2021, Black et al. 2016, Shibata and Solo-Gabriele 2012). Seaweed is usually composed of seagrass and macroalgae (Macreadie et al. 2017). Historically studies have recognized that among seaweed, macroalgae bioaccumulate arsenic from oceanic waters (McGillicuddy et al. 2023). Lately two species of macroalgae, Sargassum fluitans and Sargassum natans herein referred to as sargassum, has been gaining attention due to its increased dominance in the Atlantic Ocean (van Tussenbroek et al., 2017). Although sargassum grows in the pelagic waters of the Atlantic Ocean, depending upon ocean currents and winds, it may strand onshore at recreational beaches, providing a route of exposure to arsenic.

With shifts in climate and other anthropogenic phenomena, the amounts of sargassum strandings have been increasing along the shores of the global oceans and in particular the Caribbean and the eastern coast of the US (Tonon et al. 2022, Tomenchok et al. 2021). Ranges of accumulated arsenic in sargassum observed globally are 18.3 mg/kg to 250 mg/kg (Alleyne et al. 2023, Alleyene, 2023, Collado-Vides et al. 2020, Rodríguez-Martínez et al. 2020, McGillicuddy et al. 2023). However, there is some uncertainty surrounding the source of arsenic in the ocean. In addition to its natural ability to accumulate arsenic, arsenic availability to sargassum is believed to arise from a combination of natural processes and human activities, including historical pesticide use (Rosiana et al., 2022) and its use for the preservation of wood (Jones et al., 2019; Khan et al., 2004 & 2006). This natural bioaccumulation of arsenic by the sargassum, creates a new issue as the arsenic contaminated sargassum is washed on shore, where it can pose risks to those who work or play in beaches.

Very few studies are available that examine children’s beach behavior coupled with arsenic concentrations in the beach environment to address potential exposure and risks. Children’s beach behaviors have been assessed in the Beach Exposure and Child Health Study (BEACHES) (Ferguson et al. 2019, 2020, 2021).

Through BEACHES, Ferguson et al. (2021) conducted videotaping and video translation of children’s play activities, along with parental surveys to document both micro-activities (e.g., times child hands touch mouth) and macro-activity factors (e.g., amount of time spent at beach) relevant for exposure and risks to contaminants at beaches (Ferguson et al. 2019).

In addition, a recent study has been completed documenting arsenic levels at beaches in southeast Florida within beach water, sand and sargassum (Abdool-Ghany et al. in review). Coupled together, these studies provide a rich source of data for conducting an arsenic risk assessment for children who play at beaches impacted by stranded sargassum.

Arsenic can be toxic at low levels of exposure (Devault 2022, 2021). Health effects from arsenic range from acute symptoms to chronic conditions, including cancer (Marquez et al. 2011, Abdul et al. 2015, Beamer et al. 2016). Non-cancer acute and chronic illnesses include gastroenteritis, hematuria, patchy hyperpigmentation in the skin, eczematoid lesions, and alopecia (Ramirez-Andreotta, et al 2013). Arsenic is associated with cancers of the skin, lungs, and bladder (Kuivenhoven & Mason 20; Hong et al. 2014).

The vulnerability of different age groups to arsenic exposure varies, with infants, children, and adolescents considered more susceptible due to their developing physiology, higher metabolic rates, and behaviors such as hand-to-mouth activity that increase ingestion risks (Ferguson & Solo-Gabriele 2016; Ferguson et al. 2017). This heightened susceptibility is particularly concerning given evidence of dietary arsenic exposure through contaminated food crops and animal products, where inorganic arsenic species, prevalent in sargassum leachates, pose significant carcinogenic risks, especially in young populations (Nachman et al. 2013, Nachman et al., 2017a, Nachman et al. 2017b). Additionally, the presence of pica behavior, characterized by the consumption of non-food items, may exacerbate exposure risks, particularly in children (Miao et al. 2015, Shibata et al. 2016). Understanding the interplay between age-specific vulnerabilities, pica behavior, and multiple exposure pathways is essential for a comprehensive risk assessment and for effective mitigation strategies.

In this context, research is needed to evaluate the risks to public health from arsenic in sargassum strandings. The objectives of this study were to assess risks associated with exposure to arsenic at beaches across different age groups, spanning from infancy to adulthood. The risk assessment considered non-dietary ingestion, dermal, and inhalation exposures through contact with beach water, sand, and sargassum. The risk assessment also included the consideration of risk for a subset of children who exhibit pica behavior. To confirm the bioavailability of arsenic from sargassum, fresh and dried sargassum was analysed in this study using the Synthetic Precipitation Leachate Protocol (SPLP) to evaluate whether the arsenic is easily released and can thereby serve as an exposure route through contact in the natural environment. Results from this study can be used to inform the development of strategies to mitigate exposure and protect vulnerable populations.

This research employed the elements of the National Research Council (NRC) risk evaluation framework, encompassing hazard identification, exposure analysis, dose-response evaluation, and risk characterization (Altomare et al. 2021, NRC 1983).

In the context of this paper, the hazard identified, arsenic, has become a recent concern, due to recognition that it bioaccumulates in sargassum and that sargassum is stranding on coastal beaches at increasing rates resulting in potential increases in exposures to populations that recreate at beaches. Arsenic concentrations at beaches were determined from measurements in water, sand and sargassum conducted during three separate visits to five beaches in southeast Florida (Abdool-Ghany et al., in review). In brief, for water, the methods for this prior study involved collecting ankle-deep water samples along the shore, with results indicating that arsenic concentrations were below the detection limit of 30 µg/L. Sand samples were collected from areas both under the sargassum seaweed and higher up on the shore in the supratidal zone with concentrations ranging from 0.58 to 4.9 mg/kg. sargassum samples when available were collected directly from the location stranded along the shore and had concentrations between 9.9 to 64.3 mg/kg (Fig. 1).

The arsenic concentrations in the sand and sargassum exceeded the Florida Soil Clean Up Target Levels (SCTLs) established for arsenic in residential applications (2.1 mg/kg, FDEP, 2005). The sand was within the industrial guideline level (12 mg/kg, FDEP, 2005). The arsenic levels observed in sargassum samples exceeded both residential and industrial SCTLs. The maximum levels of arsenic observed in sargassum was near the US Environmental Protection Agency’s guideline level for land application of biosolids (75 mg/kg). The established Florida SCTLs were specifically designed to assess potential health risks associated with soil contaminant concentrations, with the residential SCTL tailored to evaluate child exposure in residential settings and the industrial SCTL intended for assessing arsenic exposure by adults in occupational environments (FDEP 2005). Given that arsenic levels in sargassum exceed the established SCTLs coupled with the proximity to the biosolids standard, the risk assessment specific to children’s recreational activities at beach sites was warranted.

To evaluate whether the arsenic in sargassum is released easily, this study sought to evaluate the transfer of arsenic from fresh and dried sargassum by SPLP (USEPA., 1994). To accomplish this, freshly stranded sargassum samples were collected from a south Florida beach (21.71917N ֯ 80.14835W ֯) on three separate days (2nd, 18th and 30th of July 2024). For each fresh sample collected, it was split three ways: one for total arsenic analysis of the solid sargassum tissue (called FS for “fresh solid”), one for SPLP (called FL for “fresh leachate”), and one to prepare a dried sargassum sample. The dried sample was prepared by placing 500 g of fresh sargassum into a mesocosm overlying a 2.5 cm layer of sand which was designed to allow for natural rainfall and solar radiation exposure. The rainwater could freely drain through the bottom of the sand layer. After two weeks, the corresponding dried sargassum samples were harvested and split with one split used for the analysis of total arsenic (called DS for “dry solid”) and another split used for SPLP analysis (called DL for “dry leachate”). See Fig. 2 for a diagram illustrating how each sample was processed.

The SPLP analysis for the fresh and dry samples followed standard protocols (FDEP 2009). In brief, 100 g of sargassum sample were sifted (Using a 9.52mm U.S. Standard Sieve Coarse Series A.S.T.M. E-11 Specifications) and placed into pre-labeled bottles containing two liters of deionized (DI) water which had been adjusted to pH 4.2 with a 60:40 solution of concentrated sulfuric and nitric acids. A blank sample was prepared with an additional 100 mL of DI water. Samples were tumbled for 18 hours, the leachates were filtered (0.7 µm nominal pore size glass fiber filters), and pH measurements were recorded (See Table S-7 for values). Filtered leachates were stored at 4°C for total arsenic analysis.

Total arsenic levels in all samples were determined using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) with a Perkin Elmer NexION 2000 instrument, adhering to the laboratory standard operating procedures (SOPs) modified after established protocols (Method 6020B, US EPA, 1994a). The liquid leachate samples were acidified with nitric acid (HNO₃) to have an acid concentration of 2% HNO₃ (v/v) before analysis. For solid samples, preparation involved pulverizing the material into a fine powder using a Cuisinart coffee grinder, followed by processing a 0.5 g portion through the digestion procedure (Method 3050B, US EPA, 1996),. The digestion was done by adding 5 mL of concentrated HNO₃ and heating the sample on a hot block (Model 200, Environmental Express) for 1 h at 95 ± 5 ˚C. The samples were then let to cool to room temperature, and 1 mL of 30% H₂O₂ was slowly added into the digestion tube and the sample was heated again for another 20 minutes at 95 ± 5 ˚C. The samples were left overnight for particulate to settle down and an aliquot of digestate was then pipetted out and diluted for analysis. Certified reference materials (CRMs, Spex CertiPrep CLMS-2N and GBW10043 for liquid and solid, respectively) were treated as samples and included accompanying each analytical batch. The analysis on ICP-MS was done using external calibration and incorporating yttrium as an internal standard to check instrumental shift.

The fraction of arsenic transferred from the sargassum to the SPLP leachate was determine from mass balance considerations. The mass of the arsenic originally in the sargassum, M_{As_S}, was the product of the arsenic concentration of the sargassum (mg arsenic per kg dry weight of sargassum) and the mass of sargassum (dry weight of the 100 mg added) used for experimentation. The mass of the arsenic transferred to the leachate, M_{As_L}, was the product of the arsenic concentration of the leachate and the volume of the water used in the SPLP experiment (2L). The fraction leached was therefore M_{As_L}/M_{As_S}.

The ability of arsenic in sargassum to transfer to a liquid similar to rainwater is an indication of how bioavailable it is. In this study, we found that a significant proportion of arsenic transfers from the solid to the liquid phase during extraction, with extracted amounts reaching up to 54% of the total arsenic present in the solid phase. This aligns with findings from prior studies which have reported ranges of total arsenic concentrations in sargassum between 10–100 mg/kg, with leaching percentages varying from 50–80% depending on the environmental conditions (Thouin et al., 2024). Our results showing extraction rates for total arsenic species ranging from 6–54%, are consistent with these observations. The substantial transfer of arsenic suggests its potential bioavailability in aqueous environments, which may influence exposure risks.

The results of this study present an initial estimate of possible health risks associated with arsenic exposure to children from recreational beach play with water, sand and sargassum. Given the limited reference values available for non-cancer risks (available for oral routes only), no non-cancer risks can be computed for other routes. Future work is needed to establish Minimum Risk Levels for dermal and inhalation routes of exposure before non-cancer risks can be further explored.

For cancer, the calculated risks, particularly the mean values, generally fall within the “extremely low” or “very low” increased risk categories, according to the interpretation in Table 3. The threshold of 10^− 6, often used as a benchmark for negligible risk, is met across most exposure routes and age groups, but exceeded when considering lifetime exposures. For individuals who visit a beach throughout their lifetime, results suggest that the overall health risks associated with arsenic exposure may be considered to exceed the 10^− 6 benchmark with risks found at 10^− 4 suggesting a low increased risk of cancer.

The study identified pica behavior as a significant factor increasing oral exposure risks. While risks from oral exposure alone were generally low, the inclusion of scenarios with pica demonstrated a noticeable increase in risk, especially for the age group 3 to 6. This finding highlights the importance of considering children’s behavior in risk assessments and calls for targeted interventions and awareness programs to reduce pica behavior during beach play. Ingestion rates and relative bioavailability were the driving factors for oral risk results. Ingestion rates were 5000 mg/day, for pica children and 60 mg/day for non-pica children. The rates for children with pica were greater by a factor of 83, therefore driving the increased risk. Relative bioavailability was determined from existing literature where mammals with similar human physiology were tested to determine the average value of gastrointestinal absorption of arsenic from soil. Results showed a mean value of 33% or 0.33. The arsenic doses (0.3 to 1.0 mg As/kg body weight) given to Cebus apella monkeys in the study (FDEP 2003) exceeded typical human dietary intake levels, suggesting the need for lower doses to accurately measure arsenic bioavailability from soils for humans. While prior research has not shown a dose-dependent effect on absorption, a comprehensive exploration across relevant dose ranges is lacking and more research is needed to characterize relative bioavailability for arsenic exposures from sargassum.

Dermal exposure showed the highest risks among all exposure routes, emphasizing its importance in contributing to the overall health risks. The results indicate a gradual increase in risk with age, reaching a maximum of 4.84×10^− 4 for lifetime risks. This underscores the need for a more in-depth exploration of dermal exposure pathways and absorption factors, especially given the absence of minimum risk levels (MRLs) specifically for dermal exposure routes. The driving role of dermal exposure in older age groups suggests that protective measures, such as enhanced hygiene practices and beach management, may be necessary for mitigating health risks. Higher dermal exposure for adults was driven by the adsorption factor, and longer duration of exposure when compared with all other age groups; the longer an individual is exposed to arsenic the greater the risk. The absorption factor contributed significantly to higher risks associated with dermal exposure. In our study, we chose to utilize a dermal absorption factor of 3.9% (CEPA 2012). Notably, the pivotal in vivo monkey study conducted by Wester et al. (1993a) reported an average fractional absorption of 3.9% or 0.039 for arsenic, based on experiments where arsenic was directly applied to soil before skin contact with the soil. This value contrasts with the assumptions made in the FDEP study, which relied on default values of 0.01 for organics and 0.001 for inorganics, based on USEPA Region 4 guidance. While the technical basis for these defaults is not explicitly elucidated, they are purported to consider reduced dermal absorption from soil. However, evidence suggests that some chemicals may exhibit higher dermal absorption rates, as indicated in the USEPA Dermal Assessment guidance. The decision to use a different value (3.9%) is based on the specific characteristics of the soil in question, particularly its grain size and composition. Sandy soils, such as those found on beaches, have different properties compared to other soil types. These soils may have larger particles and lower capacity to retain arsenic due to their porosity and texture (CEPA, 2012). In vitro studies show a wide range (0.3 to 10%) of total skin absorption for freshly spiked soil, while aged arsenic in soil resulted in lower skin absorption (0.8–1.5%) in pig skin samples (Skowronski et al., 1994; Abdel-Rahman et al 2005; Abdel-Rahman et al., 1999; Wester et al., 1993a). The disparity suggests that aging processes in soil may alter arsenic bioavailability, impacting dermal absorption rates.

A limitation of this study is that pulverizing the seaweed increases the availability of arsenic. It could be argued, however, that on beaches seaweed naturally breaks down into very small particles over time, making arsenic increasingly available within the sand. Another consideration is that the leachate used in this study may not perfectly replicate human sebum, sweat, or common skin additives such as sunscreen, all of which can influence partitioning and uptake across the skin barrier. In addition, absorption is a dynamic process across multiple skin layers, and activities such as bathing or scrubbing immediately after contact can alter the extent of uptake. Although our exposure assessment relied on frequency of contact rather than duration, in reality duration is also an important factor. In a beach environment this becomes especially complex, as fine sand particles containing seaweed often remain adhered to the skin, except immediately following a swim event.

Therefore, future studies are needed to understand the effects of aging on arsenic absorption from soil (Abdel-Rahman et al., 1996) and specifically the dermal absorption rates of arsenic from sargassum contact, as other factors such as thickness of the medium on the surface of the skin can also affect diffusion across skin barriers (Ferguson et al. 2009).

Considerable uncertainty exists in estimating parameters related to children's behavior, such as ingestion rates and dermal contact factors. The limited data on children's interactions with sargassum necessitates caution in extrapolating from sand data. Ferguson et al. (2021) reports low fractions of contacts that involve seaweed (< 1.5%), although the beaches in the Ferguson et al. (2021) study were either devoid of seaweed or groomed immediately prior to the study. It can also be assumed that people may avoid beaches that are overwhelmed with seaweed or sargassum, for recreational purposes, and may not be exposed to the accumulated arsenic over extended periods of time. We recommend further research to better understand how children engage with sargassum especially in terms of how it may alter exposure patterns and risks. Some research highlights the socio-economic implications of sargassum accumulation, noting that public perception and willingness to pay for seaweed management are influenced by the nuisance it poses to coastal activities, with many communities expressing concerns over its impact on tourism and fisheries (Ofori et al., 2023). This aligns with the notion that individuals, including families with children, may avoid beaches heavily impacted by sargassum, potentially reducing direct exposure to accumulated arsenic but also emphasizing the need for further research on behavioural interactions with the seaweed.

Specifically with the dermal exposure route, values for adherence factors for sargassum were not available in existing literature, and an assumption was made to utilize the adherence factor for sand for sargassum. Data for adherence factors were available for three age groups (0–3, 3–6 and, 20 and over) and were interpolated for the remaining age groups. Sargassum, being a natural organic material, undergoes decomposition processes when washed ashore. As it disintegrates, its organic matter breaks down and becomes integrated into the surrounding sand, altering its composition and characteristics. This integration process is gradual and can vary depending on environmental conditions such as temperature, humidity, and microbial activity.

As sargassum dries, the arsenic is lost and decreases in concentration (Abdool-Ghany et al. 2023a&b). This study focused on fresh and dried sargassum; however, little is known about the adherence associated with either type of sargassum. The assumption to use the same adherence factor for both sand and sargassum stems from the understanding that as sargassum disintegrates and integrates into the sand; physical properties may become more similar, thus leading to comparable adherence. However, this assumption may oversimplify the complexities of sargassum disintegration and integration processes. There may be differences in the adherence properties between sand and sargassum, particularly regarding the surface texture, composition, and presence of organic residues. Therefore, utilizing adherence factors from sand data for sargassum introduces uncertainties and limitations in exposure assessments, particularly concerning dermal exposure routes. Further research is warranted to better characterize the disintegration dynamics of sargassum and its integration into the sand, along with investigating specific adherence factors for sargassum across different age groups. This would enable more accurate and nuanced risk assessments, especially concerning children's interactions with sargassum on beaches.

The relatively low risks from inhalation suggest that it may not be a significant contributor to the overall health risks assessed in this study. Inhalation exposure, although showing an increasing trend with age, consistently showed lower risks compared to oral and dermal routes. The maximum values for lifetime risk in older age groups emphasize the potential cumulative effects over time.

Further research is needed to ascertain whether volatile gases including arsenic can be generated from decomposing sargassum, which may influence inhalation risks. The current risk assessment did not consider the possibility of volatile gases containing arsenic, which may underestimate the risks through inhalation. The elucidation of molecular mechanisms underlying microbial arsenic methylation and volatilization represents a recent advancement, although primarily documented on plants pure cultures (Di et al., 2019 & Wang et al. 2014). However, the linkage between microbial communities and bio-volatilization remains poorly understood, necessitating future studies to explore this relationship along with environmental factors. The contribution of arsenic gas volatilization from decomposing sargassum to inhalation risks warrants further investigation.

This study encountered several limitations that warrant consideration in interpreting the findings and guiding future research efforts. Firstly, the absence of established Minimum Risk Levels (MRLs) for dermal exposure to arsenic poses a challenge in assessing non-cancer risks associated with arsenic exposure through skin contact with contaminated sargassum. Without reference levels for dermal exposure limits, uncertainties remain regarding the potential health effects from arsenic absorption via this route. Similarly, the lack of established MRLs for inhalation exposures also eliminates the ability to assess non-cancer risks from inhalation routes. This coupled with the lack of data about the potential for arsenic volatilization from sargassum, greatly limits the ability to assess risks through inhalation.

The Monte Carlo Simulation used to assess cancer risks is based on assumptions which can vary from a real-life scenario, particularly in the instances of children behaviour and contact patterns. Furthermore, the lack of arsenic speciation data presents a significant limitation in understanding the health risks linked to sargassum exposure. Speciation analysis is crucial to discern whether the arsenic accumulating in sargassum is organic or inorganic. Given the differing toxicity and bioavailability of these forms of arsenic, the absence of this information hampers a comprehensive assessment of the potential health implications of arsenic exposure from sargassum. Additionally, the study did not explore the absorption of methylated forms of arsenic through dermal pathways. Methylated arsenic compounds may exhibit distinct absorption rates and toxicities compared to inorganic arsenic. Understanding the potential absorption of these compounds through dermal routes is vital for a thorough risk assessment, particularly considering the involvement of microbial processes in arsenic methylation and volatilization. Moreover, adherence factors for both fresh and decaying sargassum are needed to assess the bioavailability of arsenic from different states of sargassum degradation. The varying degradation states of sargassum could influence arsenic absorption rates, impacting health risk assessments. Arsenic speciation analysis becomes crucial in this context to discern differences in bioavailability between organic and inorganic forms as the sargassum decomposes.

Lastly, limited research exists on organic arsenic species within the environmental matrices, presenting a gap in understanding their toxicity and bioavailability. Organic arsenic compounds may behave differently from inorganic ones and may require specific considerations in risk assessments. Further studies are needed to explore the presence and effects of organic arsenic species in sargassum and their implications on human health.

In addition to arsenic, sargassum is known to accumulate other heavy metals (e.g., lead and cadmium) through chelation and ion exchange biosorption mechanisms (Raize et al. 2004). Given the toxicity, these additional metals should also be included in future risk assessments to obtain a holistic view of risks posed by large sargassum strandings.

This study represents a first step in assessing health risks to arsenic from sargassum stranded at beaches. We recommend considerations for the development of policies to help safeguard the public (Nachman et al., 2011). This can be inclusive of signage on beaches to provide warning to the public. This can possibly raise awareness among local communities and beach visitors about the long-term potential impacts of sargassum and arsenic exposure, encouraging them to adopt safer behaviors ((Desrochers et al., 2020, ADCN 2019).

Policy implications stemming from this research highlight the necessity for collaborative efforts among stakeholders to develop and implement effective beach management plans. These plans should include seasonal monitoring programs, early warning systems, community engagement, and public awareness campaigns to mitigate health risks associated with sargassum influxes. Legislation specific to sargassum management, innovative removal methods, and international collaboration are also crucial for addressing the challenges posed by sargassum influxes.

Several limitations exist within this study, including the absence of established Minimum Risk Levels (MRLs) for dermal exposure to arsenic, lack of arsenic speciation data, and limited research on organic arsenic species. Addressing these limitations through further research and data collection focused on sargassum adherence and dermal sorption of arsenic would enhance the accuracy and reliability of risk assessments related to arsenic exposure from sargassum, ultimately informing policies and management strategies for mitigating associated health risks on beaches impacted by sargassum influxes. By gaining a deeper understanding of this issue, we can work towards safeguarding coastal communities and ensuring the safety of beachgoers, particularly those at higher risk, such as children, from the potential health impacts of arsenic contained in sargassum. Further research is essential to improve our understanding of arsenic exposure risks from sargassum, particularly through the dermal route. Studies should focus on diffusion experiments using lipid-simulating liquids to model arsenic absorption through human skin, adherence studies to assess how fresh sargassum clings to the skin, and bioavailability testing for arsenic in seaweed and sand. Additionally, investigations into children's behavior on beaches, including time spent in contact with sargassum and sand, are needed to refine exposure estimates. These efforts will enhance the accuracy of risk assessments and inform strategies to mitigate potential health impacts on vulnerable populations.