261Metalloid Section
Drinking water contaminants that can cause health effects after continuous long-term
exposure at levels greater than the maximum contaminant level (MCL) are considered
“chronic” contaminants. Examples of chronic drinking water contaminants regulated by
EPA include inorganic contaminants like arsenic, cadmium, and copper; organic
contaminants such as pesticides and industrial chemicals; and radiological contaminants
like radium and uranium.
If your water system has installed some form of inorganic contaminant or arsenic
treatment, keep in mind that the treatment you installed may change the water quality in
other ways. It might cause the water to react differently in the distribution system.
Depending on the kind of treatment you’ve installed, consider what distribution system
problems might result.
A change in the taste, odor or appearance of the water at customers’ taps may be the first
indication of a problem. Some water quality parameters to consider monitoring, depending
on your arsenic treatment technology, include iron, pH, manganese, alkalinity, and
aluminum.
In contrast, “acute” contaminants can cause short-term health effects within hours or days
of exposure. Microbes such as E. coli and Cryptosporidium are examples of contaminants
that can cause an acute health risk. Some chronic-type contaminants can also fall in this
category if they are present at high enough concentrations to cause immediate health
effects. For example, nitrate levels over the MCL can cause “blue-baby” syndrome in
children less than 6 months.
Arsenic, boron, silicon, germanium, antimony and tellurium are commonly classified as
metalloids. One or more from among selenium, polonium or astatine are sometimes added
to the list. Boron is sometimes excluded from the list, by itself or together with silicon.
Tellurium is sometimes not regarded as a metalloid. The inclusion of antimony, polonium
and astatine as metalloids has also been questioned.
A metalloid is a chemical element with properties that are in-between or a mixture of those
of metals and nonmetals, and which is considered to be difficult to classify unambiguously
as either a metal or a nonmetal. There is no standard definition of a metalloid nor is there
agreement as to which elements are appropriately classified as such. Despite this lack of
specificity the term continues to be used in the chemistry literature.
Some authors do not classify elements bordering the metal-nonmetal dividing line as
metalloids noting that a binary classification can facilitate the establishment of some
simple rules for determining bond types between metals and/or nonmetals. Other authors,
in contrast, have suggested that classifying some elements as metalloids 'emphasizes
that properties change gradually rather than abruptly as one moves across or down the
periodic table.
Alternatively, some periodic tables distinguish elements that are metalloids in the absence
of any formal dividing line between metals and nonmetals. Metalloids are instead shown
as occurring in a diagonal fixed band or diffuse region, running from upper left to lower
right, centered around arsenic.