3. Hazards and Limits in Food and Water
Inorganic arsenic (iAs) can be found in the environment in several forms, including As(0) (metalloid arsenic), As(III) (arsenite) and As(V) (arsenate). The latter two forms are abundant in natural and drinking waters 
. Arsenate is the dominant specie in oxic waters, whereas under mildly reducing conditions, the probability of arsenite prevalence increases 
. As(III) is 60 times more toxic than As(V), because of its greater tendency to react with lipids, proteins and other cellular components, causing higher cellular uptake 
. In addition, As(III) is more difficult to remove from water due to its higher mobility, as compared with As(V); therefore, it can be adsorbed less efficiently on solid surfaces 
Because of its toxicity, the World Health Organization (WHO) in 1993 reduced the recommended concentration limit of total As in drinking water to 10 μg/L (from the previous 50 μg/L limit) 
Equally important and hazardous is the exposure to As from food consumption. Arsenic can accumulate in plants, mainly in cereals, reducing their growth and productivity 
. Irrigation needs of crops are often related to the amount of arsenic content in the seeds, due to the already contaminated groundwater or/and surface water application 
. The corresponding recommended limit by Food and Agriculture Organization (FAO), in collaboration with WHO, applies currently mainly to rice grains, due to the respective higher irrigation needs, as compared with other crops, and it has been set at 0.2 mg/Kg 
4. Main Effects of Arsenic Contamination on Human Health
Arsenic is considered a top priority contaminant due to its toxicity and as a carcinogenic chemical, whereas the intake by humans has been verified worldwide, through the consumption of contaminated water and food. Both organic and inorganic forms of arsenic can exist in the environment, but the latter ones are more poisonous and toxic. Inorganic As is found more in water sources and, consequently, in the relevant edible fish products. Regarding vegetation, the exposure to arsenic contamination (e.g., through contaminated soil) can cause the inhibition of plant growth along with the loss of or reduction in photosynthetic and reproductive activities.
The accumulation of arsenic in the food web may lead to acute and long-term effects on human health. Vomiting, abdominal pain, diarrhea, numbness and tingling, muscle pain and cramps, and death in extreme cases are reported as the main health symptoms of elevated arsenic intake 
The long-term effects are attributed to higher levels of inorganic arsenic in the human body. The effects can be observed mainly in skin, including pigmentation, lesions formation and patches, and acting probably as precursors to skin cancer. Bladder and lung cancers are also reported to be a result of arsenicosis 
. Other health issues such as diabetes, pulmonary and heart diseases may also arise, due to the long-term exposure of arsenic; e.g., Taiwan has witnessed gangrene because of black-foot disease, leading to deaths, due to elevated arsenic concentrations 
5. Control, Prevention and Treatment of As in Water Sources
Regarding the prevention and control of arsenic, various measures can be proposed and applied. First, the higher or lower arsenic sources should be identified appropriately. Second, the higher arsenic-contaminated sources of groundwater should be substituted by other safer water sources, whereas the lower arsenic concentration waters can be used for various domestic purposes (probably not for dinking). Third, the higher arsenic-contaminated waters can be blended with the lower ones to achieve an average concentration, which is permissible for their respective use, according to WHO regulations. For long-term prevention and control, industrial and other wastewaters should be treated properly. The general public should be informed of the arsenicosis problem and its health effects and the population at high-risk of arsenic toxicity should be monitored regularly 
. Fourth, the necessary arsenic treatment/removal systems should be installed. These systems, such as adsorption, precipitation, coagulation, ion-exchange, membrane filtration techniques, among others, may be centralized or applied locally.
Reducing the concentration of arsenic to below the permitted concentration limits requires the application of effective methods, since the arsenic removal mechanisms may pose certain difficulties. According to the speciation diagram of this element, the dominant As(III) specie in waters is the neutral form, for which it is more difficult to apply a selective removal mechanism. A common solution proposed in the literature and employed in practice is the preliminary As(III) oxidation to the As(V) form, which is negatively charged in the water pH range usually encountered 
. Oxidation may be achieved chemically, or biologically by using the appropriate microorganisms 
On the other hand, and regarding the applied treatment technology, most of the aforementioned treatment methods present specific limitations, such as higher cost, lower selectivity, insufficient removal (i.e., the As residual concentration may be higher than the 10 μg/L concentration limit), higher energy consumption, or production of large amounts of toxic sludge. Among them, the most promising seems to be the adsorption process, since by applying the appropriate sorption materials, such as conventional iron- or aluminum-based adsorbents 
, or better yet, the novel engineered inorganic nanoparticles 
, it is possible to overcome most of the previous limitations. The adsorption-based technologies are also those that have found extensive applications in the highly affected As-contaminated areas of Southeast Asia 
. In addition, it is possible to combine the respective removal mechanism with an oxidation mechanism, producing a unique step process, and therefore, increasing the method’s effectiveness 
Arsenic concentration in food and drinking water above the maximum permissible concentration limit is a common water pollution problem in both developed and developing countries. The exposure to higher concentrations of arsenic may be life threatening. Water sources for drinking, such as surface water or groundwater, and food sources, such as fish, crops and cereals, can play a notorious role in exposing humans to arsenic. Taking into account these problems, the continuous monitoring of arsenic levels in water and food sources is mandatory in the future, allowing the application of proper treatment/removal process and the prevention of humans from arsenic intake.