263The criterion that metalloids are difficult to unambiguously classify one way or the other is
a key tenet. In contrast, elements such as sodium and potassium 'have metallic properties
to a high degree' and fluorine, chlorine and oxygen 'are almost exclusively nonmetallic.'
Although most other elements have a mixture of metallic and nonmetallic properties most
such elements can also be classified as either metals or nonmetals according to which set
of properties are regarded as being more pronounced in them. It is only the elements at
or near the margins, ordinarily those that are regarded as lacking a sufficiently clear
preponderance of metallic or nonmetallic properties, which are classified as metalloids.
Which Elements are Metalloids?
There is no universally agreed or rigorous definition of the term metalloid. So the answer
to the question "Which elements are metalloids?" can vary, depending on the author and
their inclusion criteria. Emsley, for example, recognized only four: germanium, arsenic,
antimony and tellurium. James et al., on the other hand, listed twelve: boron, carbon,
silicon, germanium, arsenic, selenium, antimony, tellurium, bismuth, polonium,
ununpentium and livermorium. As of 2011 the list of metalloid lists recorded an average
of just over seven elements classified as metalloids, per list of metalloids, based on a
sample size of 194 lists.
The absence of a standardized division of the elements into metals, metalloids and
nonmetals is not necessarily an issue. There is a more or less continuous progression
from the metallic to the nonmetallic. A specified subset of this continuum can potentially
serve its particular purpose as well as any other. In any event, individual metalloid
classification arrangements tend to share common ground (as described above) with most
variations occurring around the indistinct margins, as surveyed later.
How Are Chronic Contaminants Regulated?
In 1974, Congress passed the Safe Drinking Water Act (SDWA) to give EPA the authority
to set standards to ensure the safety of drinking water provided by public water systems.
The SDWA, which was amended in 1986 and 1996, directs EPA to establish non-
enforceable health goals called maximum contaminant level goals (MCLGs) which reflect
the level at which no adverse health effects are expected from a particular contaminant.
Once an MCLG is established, EPA sets enforceable standards for contaminants called
maximum contaminant levels (MCLs). MCLs are set as close to the health goals as
possible considering cost, benefits, and the ability of public water systems to detect and
remove contaminants using appropriate treatment technologies. When there is no reliable
method to measure a contaminant that is economically and technically feasible, EPA
develops a treatment technique requirement rather than an MCL. EPA continues to assess
the occurrence of unregulated contaminants through the Unregulated Contaminant
Monitoring Regulation (UCMR). Information about the UCMR can be found at
http://www.epa.gov/safewater/ucmr/.
What Are Some Best Practices For Effective Communication About Chronic
Contaminants?
If you expect that your public water system will exceed EPA’s standard for a contaminant
or that the costs of compliance may require public funding, communicate early and often.
The most effective communication efforts follow these simple steps:
Provide simple, straightforward, and consistent messages;