Letter to the Editor on Parker et al. 2022 “Development of Fluoride Protective Values for Aquatic Life Using Empirical Bioavailability Models”

Despite human activities have currently increased the global flux of fluoride into the atmosphere and in rivers by more than a factor of 2 (Schlesinger et al., 2020), national water quality criteria for fluoride to protect freshwater biota have been established in only a few countries. The Canadian Council of Ministers of the Environment developed an interim water quality guideline of 0.12mg F/L (Environment Canada, 2001) by multiplying the 144 h median lethal concentration (LC50) value of 11.5mg F/L for the caddisfly Hydropsyche bronta (Camargo et al., 1992) by a safety factor of 0.01. This Canadian water quality guideline for fluoride appears to be adequate to protect sensitive native freshwater animals from anthropogenic fluoride pollution. Parker et al. (2022), however, have considered the Canadian water quality guideline for fluoride to be overly conservative, developing fluoride protective values for freshwater life that are much less restrictive. Based on Pearcy et al.'s (2015) work, Parker et al. (2022) conducted a meta‐analysis of available aquatic toxicity literature for fluoride to evaluate the utility of water hardness, alkalinity, and chloride as toxicity modifying factors (TMFs) in multiple linear regression‐based bioavailability models of freshwater taxa. When considering TMFs, they developed chronic protective values ranging from 3.4 to 10.4mg F/L, while they estimated a chronic protective value of 4.0mg F/L without considering TMFs. Parker et al. (2022) concluded that suitable water quality benchmarks for fluoride would range from 3.4 to 10.4mg F/L, depending on the presence of TMFs (notably chloride and alkalinity). In contrast to Parker et al.'s (2022) meta‐analysis, Pearcy et al. (2015) conducted acute toxicity tests using juveniles of the amphipod Hyalella azteca and fry of the salmonid Oncorhynchus mykiss with varying concentrations of water hardness, chloride, and alkalinity as possible toxicity modifying factors for fluoride. They found that chloride played a major role in modifying fluoride toxicity when hardness and alkalinity were held constant: estimated 96 h LC50 values for H. azteca were 8.1, 11.0, 17.8, and 24.8mg F/L with 3, 6, 12, and 25mgCl/L, respectively, in moderately hard water (80mgCaCO3/L) and low alkalinity (20mgCaCO3/L); estimated 96 h LC50 values for O. mykiss were 27.7, 49.9, 55.1, and 90.9mg F/L with 4, 8, 16, and 32mgCl/L, respectively, in soft water (10mgCaCO3/L) and very low alkalinity (4 mgCaCO3/L). Pearcy et al. (2015) also found that alkalinity was not a toxicity modifying factor, whereas hardness did not appear to be a primary toxicity modifying factor. In addition, Pearcy et al. (2015) conducted chronic toxicity tests with varying concentrations of chloride (2, 6, and 18mgCl/L), finding that chloride also reduced fluoride toxicity in both test species: estimated 14 days LC50 values for H. azteca were 4.8, 8.6, and 12.9mg F/L in moderately hard water (88–90mgCaCO3/L), and estimated 7 days LC50 values for O. mykiss were 11.5, 25.4, and 40.9mg F/L in soft water (6 mgCaCO3/L). The ameliorating effect of chloride might be due to an intensified competition between F and Cl ions for carriers in the membranes of epithelial cells, decreasing the influx of fluoride into cells as the concentration of chloride in the aquatic medium increases (Camargo, 2004; Gonzalo & Camargo, 2012). In fact, fluoride bioaccumulation in signal crayfish significantly decreased with increasing water chloride content (Gonzalo & Camargo, 2012). Nevertheless, according to Pearcy et al. (2015), chloride did not decrease the acute toxicity of fluoride to O. mykiss in moderately hard water, and the role of chloride as a primary toxicity modifying factor for fluoride was inconsistent in chronic toxicity tests with the cladoceran Ceriodaphnia dubia and the cyprinid Pimephales promelas. In my opinion, Parker et al. (2022) have not properly considered Pearcy et al.'s (2015) findings, particularly that alkalinity was not a toxicity modifying factor for fluoride. Furthermore, after reviewing available aquatic toxicity literature for fluoride, I have not found any study showing that alkalinity is a toxicity modifying factor (see also Camargo, 2003). On the other hand, This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made.