Heavy metals contamination is a serious problem in the aquatic environment. Some of them are very important, having biological roles for aquatic organisms, and called essential heavy metals. In contrast, other heavy metals are considered harmful even at low concentrations. The toxic levels of heavy metals may be of agricultural, industrial and mining activities. This will cause water pollution and changes in the physicochemical characteristics of the aquatic environment. This pollution has deleterious toxic effects on fish and raises concerns over its potential impact on human health. The most common heavy metals are arsenic, chromium, lead and mercury, which affect human health and are considered systemic toxicants. These metals induce organ damage even at low levels of exposure and according to the US Environmental protection agency and international agency for research on cancer, they classified as carcinogens. For all the above reasons, this review was written to contribute to heavy metals' role in the environment, toxic mechanism and toxic effects on fish.
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