TCEQ Public Drinking Water Section, rev: August 12, 2005
Trihalomethanes (THMs) and haloacetic acids (HAAs) form when
disinfectants are added to drinking water systems to kill
potentially dangerous microorganisms. The added disinfectants react
with naturally occurring organic chemicals in the water to form
other chemicals, including THMs and HAAs. Because long-term
exposures to these chemicals in our water may result in adverse
health effects, EPA has established standards (the MCLs) for them.
When they exceed their respective MCLs in drinking water, your
provider is required to notify you. Notification is not intended to
suggest that you or your family members will be harmed by the
detected levels, but instead is meant to keep you informed.
Exceedance of MCLs also informs the water supplier that action is
warranted to reduce the concentrations of those chemicals in the
water system.
The required public notice language for these violations
includes statements like:
- This is not an emergency.
- You do not need to use an alternative water supply.
- Some people who drinking water containing these chemicals in
excess of the MCL over many years may have an increased risk of
cancer (HAA and THM) or some other long-term health effect (THM
only).
When EPA establishes the MCL for a chemical that is known or
suspected to cause adverse health effects from long-term exposures,
it assumes that the people who drink that water consume two liters
(about half a gallon) of it every day for seventy years
(approximately one lifetime). MCLs also are set at levels that are
expected to protect susceptible groups in our population, for
example, children, pregnant women, the elderly, and people who may
have existing health problems. For chemicals that may cause cancer,
EPA also considers what amount of the chemical would cause an
increased risk of one (1) case in ten thousand (10,000) people who
are exposed over their lifetime.
It is unlikely that the short amount of time (relative to
seventy years) that customers will drink the water with high THMs
and HAAs should cause any adverse effect on their health. EPA has
identified THMs and HAAs as a long-term health risk, not a
short-term health risk. The greater risk for short-term noncancer
health problems to children and adults, including severe
conditions, would be from the microorganisms that are killed by the
added disinfectants from which THMs and HAAs are produced.
There is no imminent risk to the health of customers.
