2.3. Health risk assessment method

Humans may be exposed to MCs both directly and indirectly. Consumption of drinking water is a main direct route, and indirect exposure include the consumption of MCs-containing freshwater fish and shellfish, contaminated crops, vegetables that have been irrigation with MCs contaminated water, other food of an animal origin, and food supplements.⁴⁷

Risk assessments are usually divided into carcinogenic and non-carcinogenic risks according to the carcinogenicity of the pollutant of interest. At present, most studies on the carcinogenicity of MCs are still in the exploration stage, and no carcinogenic intensity coefficient of MC-LR has yet been declared by the EPA.⁴⁸ Therefore, this study only considers the oral intake of MC-LR through drinking water consumption. The hazard quotient (HQ) was used to assess the noncarcinogenic risks of MC-LR to humans. It is generally believed that the response of an organism to non-carcinogenic substances has a dose threshold, and it is considered that there is no adverse health effect if the risk is below the threshold. HQ is defined as the ratio of the long-term daily intake dose to the reference dose due to exposure. The single toxicant non-carcinogenic health risk assessment model recommended by the US EPA was used for evaluation.⁴⁸ The calculation formula is as expressed by eqn (1):²³

where CDI is the long-term daily intake (oral or inhalation) dose (of a pollutant) (μg per kg per d) and RfD is the maximum exposure reference dose (μg per kg per d), which is available from the US EPA^48,49 and Integrated Risk Information System (IRIS, https://www.epa.gov/iris). When HQ is less than 1, the non-carcinogenic health risk level is considered to be acceptable; otherwise, a non-carcinogenic health risk is considered present.⁵⁰

The CDI for oral and inhalation exposure can be calculated using eqn (2):

where C_s is the content of the contaminant of interest in a medium (e.g., water, food, or air) (μg kg⁻¹ or μg L⁻¹); IR is the intake of water, food, or air (mg per d or L per d); EF is the exposure frequency (365 d year⁻¹); ED is the exposure period (non-carcinogens, 30 years; carcinogens, 70 years); BW is the average body weight (kg); AT is the average exposure time (non-carcinogens, 30 × 365 = 10 950 d; carcinogens, 70 × 365 = 25 550 d).

This article only considers the risk of MCs to human health through drinking. Therefore, the formula for calculating the non-carcinogenic risk can be simplified as eqn (3):

where C is the concentration of MCs in drinking water (μg L⁻¹); IR is the drinking water intake (L per d); RfD is the reference dose for oral exposure of MC-LR.