HARMFUL ALGAL BLOOMS AND DRINKING WATER
SUMMARY
Freshwater harmful algal blooms (HABs) are a growing
concern in the United States and worldwide. Negative
impacts from HABs on water quality, human and animal
health and the economy can be significant. Some HABs
can produce toxins that are harmful to humans and
animals. These toxins can pose challenges to drinking
water supplies. Given this risk, many drinking water
systems are taking actions to manage cyanotoxins in
drinking water and notify the public if toxin levels become
a possible health concern. Reducing nutrient pollution,
such as excess nitrogen and phosphorus, in drinking water
sources is important for the long-term management of the
risks HABs pose to public health and water quality.
BACKGROUND
Cyanobacteria, formerly referred to as blue-green algae,
are found naturally in lakes, rivers, ponds and other
surface waters. When certain conditions exist, such as in
warm water containing an abundance of nutrients, they
can rapidly form harmful algal blooms (HABs) (see Figure
1). Some HABs are capable of producing toxins, called
cyanotoxins, which can pose health risks to humans and
Figure 1. Harmful algal bloom
animals. Additionally, HABs can create taste and odor
problems in drinking water, such as an earthy and musty
smell. The environmental conditions that cause HABs to
produce cyanotoxins are not fully understood and can
vary from year to year within the same waterbody. Some
cyanotoxins occur in blooms that look like thick scum or
paint -like substances on the surface of the water, while
others occur in blooms that are not as easily visible.
HEALTH IMPACTS
Conventional water treatment (consisting of coagulation,
sedimentation, filtration and chlorination) can generally
remove cyanobacterial cells and low levels of toxins.
However, water systems may face challenges providing
drinking water during a severe bloom event, when there
are high levels of cyanobacteria and cyanotoxins in
drinking water sources. If cyanotoxins over the U.S.
Environmental Protection Agency's national 10-day Health
Advisory level (see Table 1) occur in tap water, people are
at risk of various adverse health effects including upset
stomach, vomiting and diarrhea as well as liver and kidney
damage.
10-DAY HEALTH ADVISORIES
LEVEL
Microcystins
Children pre-school age and younger
(under 6 years old)
0.3 pg/L
School-age children (6 years and older)
1.6 pg/L
Cylindrospermopsin
Children pre-school age and younger
(under 6 years old)
0.7 pg/L
School-age children (6 years and older)
3.0 pg/L
Table 1. U.S. EPA's National 10-Day Health Advisories
*>EPA
United States
Environmental Protection
Agency
Co-Branding (optional)
EPA: 810-F-16-006

-------
MANAGING HEALTH
RISKS
Drinking Water Health Advisories
Given the health concerns
that can occur from
cyanotoxins in drinking
water, many water systems
are taking actions to
manage and reduce the
risks from cyanotoxin
contamination in drinking
water. These actions can
include steps for cyanotoxin
monitoring, adjusting
treatment to address
contamination before
levels are of concern and
notifying the public through
a Drinking Water Advisory
when toxin tap water levels
are a possible public health
concern.
DRINKING WATER
ADVISORIES
ADVISORY LEVEL
Cyanotoxins detected
in tap water at levels
of concern.
Cyanotoxins detected
in tap water at levels
of concern for young
children and vulnerable
populations.*
Cyanotoxins not
detected in tap water
at levels of concern.
ACTION
I
Vulnerable populations = infants,
with pre-existing liver conditions,
Figure 2. Drinking Water Health Advisories
If a Drinking Water Advisory
is issued for cyanotoxins,
instructions for appropriate
customer actions will be
described in the advisory notification. There is a possibility
that there will be different instructions for different
population groups, depending on the cyanotoxin levels
found in the drinking water.
Using the the U.S. Environmental Protection Agency's
national Health Advisory levels for microcystins and
cylindrospermopsin as a guide, ranges of cyanotoxin levels
in drinking water can be shown with the following advisory
levels: green,	and red (see Figure 2). Green
corresponds to drinking water toxin levels where adverse
health impacts are unlikely to occur for everyone,
indicates drinking water toxin levels where the risk of
adverse health impacts is higher for infants, young children
under the age of six and other vulnerable populations
(including: pregnant women, nursing mothers, those
children under the age of six, pregnant women, nursing mothers, those
those receiving dialysis treatment, the elderly and sensitive populations.
with pre-existing liver conditions, those receiving dialysis
treatment, the elderly and other sensitive populations).
Red indicates drinking water toxin levels above which
the risk of adverse health impacts is higher for everyone
drinking the water. Drinking water systems can elect to
issue Drinking Water Advisories using these categories as
guides.
HEALTH ADVISORIES
The U.S. Environmental Protection Agency (U.S. EPA)
published national drinking water Health Advisories for the
cyanotoxins microcystins and cylindrospermopsin (see
Table 1). The Health Advisories provide the cyanotoxin
levels in drinking water less than or equal to which adverse
human health impacts are unlikely to occur when exposed
to these levels over a 10-day time period. The Health
s»EPA
United States
Environmental Protection
Agency
EPA: 810-F-16-006
Co-Branding (optional)

-------
Advisories are lower for infants and young children under
the age of six because they drink more water relative to
their body weight as compared to adults and children six
years and older.
Health Advisories are developed to help states and water
systems assess local situations and during emergency
situations and spills. They are not a federally enforceable,
regulatory limit. As new information becomes available,
the U.S. EPA may develop updated advisories. For more
information please see: https://www.epa.gov/nutrient-
policy-data/drinking-water-health-advisory-documents.
Data for illnesses associated with exposure are being
collected nationally at the Centers for Disease
Control and Prevention (CDC). To report a cyanotoxin-
associated illness for humans and animals, please contact
your state or local health department.
PREVENTING HABS
Keeping the lakes and rivers that supply our drinking
water clean is key to ensuring clean drinking water.
Reducing the amount of nutrients, such as nitrogen and
phosphorus, in sources of drinking water can reduce risks
of HABs and associated cyanotoxins impacting drinking
water. These excess nutrients typically originate from
agricultural, industrial and urban sources as well as from
atmospheric deposition. Decreasing this nutrient pollution
will help keep drinking water clean and can generally
improve local water quality (see Figure 3).
MORE INFORMATION
For more information about how HABs are managed in
your tap water contact your public water system. For
more general information see: www.epa.gov/cyanohabs.
For more information about HAB-associated illnesses,
see www.cdc.gov/habs.
Figure 3. Lake without a harmful algal bloom (Lake Crescent, WA)
*>EPA
United States
Environmental Protection
Agency
Co-Branding (optional)
EPA: 810-F-16-006

-------