Efforts to establish water quality standards for the protection of human health involve a calculation of the amount of fish that can be consumed without exceeding a predetermined safe dose. This is often accompanied by a calculation of the concentration of the substance of concern (i.e., metal) in water that should not be exceeded, to prevent excessive accumulation of the metal in the food organism. Such an exercise was considered in Europe to identify a priority substance for the Water Framework Directive and was applied to selenium in prioritization efforts by the Joint Research Centre (JRC; Van Vlaardingen and Verbruggen 2007).

The approach proposed for use by the JRC to calculate a safe water concentration or predicted no‐effect concentration (PNEC) begins with a standard methodology (equation) for back‐calculating from a safe human dose to a concentration in water that ensures the fish tissue concentration is acceptable. The standard approach is as follows:

(1)

where QS‐biota is the quality standard for biota; TDI_‐HH representes the tolerable daily intake for human health (µg/kg body wt/d; derived from a no‐observed‐adverse‐effect level [NOAEL] in an animal study divided by a safety factor of ~100); 0.1 = percentage of diet that is fish (%/d); 70 kg = average adult weight; and 0.115 = daily fish consumption.

(2)

where BAF is the bioaccumulation factor.

The output from the above calculation gives concern because it can result in water concentrations that approach or are below background. To demonstrate, simulated PNEC calculations are derived for copper, chromium, iron, selenium, and zinc (Table 1). Because the percentage of diet and daily consumption values are fixed, the final water concentration is driven by the TDI value and the BAF. The TDI for most substances has been determined by regulatory bodies, and hence the BAF becomes the driver for the output. The BAF values we selected span a range of 500 to 10 000 and are fairly typical for these metals, although larger values have been reported. However, it is well accepted that BAFs are inversely related to exposure concentration and vary by several orders of magnitude depending on exposure (McGeer et al. 2003; DeForest et al. 2007). Selecting one BAF to represent all water bodies and all species will not accurately reflect the breadth of the distribution of BAFs for natural waters. This would necessitate the selection of a conservative BAF to ensure that fish are safe to eat regardless of location. The use of Equation 1 as a universal approach for deriving a national or state PNEC value or water quality standard for regulation would necessarily result in a low value, to be protective of all water bodies.

The approach just described does not consider that many metals (those selected) are essential elements for fish and human health. Essential metals like copper and selenium are homeostatically regulated by fish to meet their metabolic needs. Hence, only minor changes in fish tissue concentrations result from large changes in the water concentrations. A further limitation of this approach is that fish obtain only a portion of their metal nutritional requirement via water exposure. For essential elements, the diet is also important. For selenium, the diet is the major uptake route. Hence, a backward calculation from fish tissue to a water concentration has serious limitations.

In the example calculations with the selected BAFs, it can be seen that use of the described modeling approach for deriving PNEC values would result in values that are below the current US Environmental Protection Agency water quality criteria (WQC) with the exception of copper and a BAF of 500 (Table 1). For these calculations, the maximum TDI was used. If a lower value such as a reference dose were to be used, the PNEC values would become even smaller.

Alternatively, fish advisories are commonly used to advise the public on consumption of fish with elevated levels of a metal. The advisories are specific to a fish species and site and thus incorporate the bioavailability of the metal at a given site. We advocate that this is a better approach to protecting the public. The WQC derived using one BAF and the previous approach will lead to PNEC values and WQC that will either under‐ or overprotect human health.

为保护人类健康而制定水质标准的工作包括计算在不超过预定安全剂量的情况下可以食用的鱼类数量。这通常伴随着计算水中不应超过的相关物质（即金属）的浓度，以防止金属在食物有机体中过度积累。在欧洲，这种做法被认为是确定水框架指令的优先物质，并在联合研究中心（JRC；Van Vlaardingen 和 Verbruggen 2007）的优先排序工作中应用于硒 。

JRC 建议用于计算安全水浓度或预测无影响浓度 (PNEC) 的方法开始于标准方法（方程），用于从安全的人体剂量反算到可确保鱼组织的水中浓度浓度可以接受。标准方法如下：

(1)

其中 QS-biota 是生物群的质量标准；TDI _‐HH代表人类健康的每日可耐受摄入量（μg/kg body wt/d；来自动物研究中未观察到的不良反应水平 [NOAEL] 除以 ~100 的安全系数）；0.1 = 鱼类饮食的百分比 (%/d)；70 公斤 = 成人平均体重；0.115 = 每日鱼类消费量。

(2)

其中 BAF 是生物蓄积因子。

上述计算的输出令人担忧，因为它可能导致接近或低于背景的水浓度。为了证明这一点，我们对铜、铬、铁、硒和锌进行了模拟 PNEC 计算（表 1）。由于饮食和日消耗值的百分比是固定的，最终水浓度由 TDI 值和 BAF 驱动。大多数物质的 TDI 已由监管机构确定，因此 BAF 成为输出的驱动因素。我们选择的 BAF 值范围在 500 到 10 000 之间，对于这些金属来说是相当典型的，尽管已经报道了更大的值。然而，众所周知，BAFs 与暴露浓度成反比，并且根据暴露情况有几个数量级的变化（McGeer 等人，  2003 年）; 德福雷斯特等。 2007 年）。选择一个 BAF 来代表所有水体和所有物种将不能准确反映自然水域 BAF 分布的广度。这将需要选择保守的 BAF，以确保无论位置如何，鱼都可以安全食用。使用公式 1 作为导出国家或州 PNEC 值或水质标准的通用方法，必然会导致较低的值，以保护所有水体。

刚刚描述的方法没有考虑到许多金属（选定的金属）是鱼类和人类健康的基本元素。铜和硒等必需金属受鱼类体内平衡调节以满足其代谢需求。因此，水浓度的大变化只会导致鱼组织浓度的微小变化。这种方法的另一个限制是，鱼类只能通过接触水来获得其金属营养需求的一部分。对于必需元素，饮食也很重要。对于硒，饮食是主要的摄取途径。因此，从鱼组织到水浓度的反向计算具有严重的局限性。

在所选 BAF 的示例计算中，可以看出，使用所描述的建模方法来推导 PNEC 值将导致低于当前美国环境保护署水质标准 (WQC) 的值，但铜和BAF 为 500（表 1）。对于这些计算，使用了最大 TDI。如果使用较低的值，例如参考剂量，则 PNEC 值会变得更小。

或者，鱼类咨询通常用于建议公众食用金属含量升高的鱼类。这些建议是针对特定鱼类和地点的，因此包含了特定地点金属的生物利用度。我们主张这是保护公众的更好方法。使用一个 BAF 和之前的方法得出的 WQC 将导致 PNEC 值和 WQC 对人类健康的保护不足或过度。