**3. Toxicity**

Heavy metals can produce side effects on soil, on water, on air, but also on plants, animals and humans [3, 4, 17]. In soil, high levels of heavy metals can produce

alteration of soil quality through modification of pH, color, porosity and natural composition [4, 18], but also low crop production, loss of many types of normal flora and habitat [19]. Their accumulation into the water imposes serious problems on humans and ecosystems [4], due to decreasing of drinking water quality and purity, decreasing water supplies for all living organisms [19]. High levels of heavy metals in air can lead to harmful health problems, including respiratory infections, cardiovascular disease, premature mortality, eyes and skin irritation, but also can cause infrastructure deterioration, acid rain increasing, corrosion, eutrophication and haze [4], low yields of the crop, not enough oxygen [19]. In plants, they can produce damage of roots or leaves, interfere in important biochemical process, such as photosynthesis, alteration of minerals absorption, damage of chlorophyll, reduce the growth and development of the roots, which leading to reduction pf overall growth of the plant [3, 20, 21].

The toxicity of heavy metals in animal is manifested through decreased body weight, kidney damage, liver affections, shortened life span, increased oxidative stress, modifications of cells composition, DNA damage [17]. In humans they can produce kidney damage, liver affections, pulmonary effects, several types of cancer [3].

Heavy metals became toxic when are not metabolized by the body and accumulates in organs and soft tissues [4]. They reach the human body by ingesting contaminated water or food, inhalation of absorption through the skin. Among the pathways, ingestion in the common route that helps the heavy metals to enter to the animal bodies [3, 4]. The effect of this metals can be inhibitory, stimulatory and toxic for some biochemical processes [3], being able to produce various health problems on nervous system (Alzheimer, Parkinsoma, depression, dementia), on bone system (bone mineralization) an on reproductive system. Also, can produce DNA damage, RNA affection, or cancer of lungs, skin, bladder, due to production of ROS [3]. Their toxicity depends the dose of exposure, time of exposure, pollutant concentration, organism which are exposed to it, nature and oxidation state of the metal [3, 4].
