The most important task in operating a swimming
pool or hot tub is maintaining
a safe environment for users.
This entails managing a variety of
hazards concurrently such as limiting
access to authorized users, elimination
of suction and entrapment hazards,
ensuring that all appropriate safety
equipment is continuously available,
prevention of unauthorized access to
treatment chemicals, and the prevention
of Recreational Water Illnesses (RWIs).
This article is the first installment in a
two-part series on RWIs. Part one will
discuss infection prevention. Part two
will discuss illnesses from exposure to
disinfection byproducts and improper
water balance.
Infectious RWIs are illnesses caused
by exposure to improperly treated water,
whether through swimming, soaking
or inhaling water mist or vapors. For
the last few decades, a spotlight has
shined brightly on those germs that
cause gastrointestinal illness. Among
these pathogens are E. coli, Shigella,
Giardia, Cryptosporidium (Crypto) and
Norovirus. Every year, there are reports
regarding cases of gastrointestinal illness
stemming from public swimming pools.
However, a recent article from the CDC
extrapolated, from reported data, that
of the estimated 5.61 million illnesses
associated with recreational water, about
80% of these illnesses were due to
swimmer’s ear (otitis externa) while 19%
of illnesses were gastrointestinal.1 The
leading cause of swimmer’s ear is the
bacterium Pseudomonas aeruginosa. All
the germs that cause RWIs, except for
Crypto, are readily controlled by 1 ppm
of free chlorine. The number and type
of RWIs demonstrates the importance
of maintaining an adequate disinfectant
concentration.
It is not possible to simply look at
the water and see if there are any germs
present. The smallest microbe that a
human can see is about 0.1 mm long.
The largest germ that causes RWIs is the
Giardia cyst, and it is 10 to 15 um long.
In other words, the Giardia cyst is about
seven times smaller than humans can
see. Cryptosporidium oocysts are half
the size of Giardia cysts, and bacteria are
a tenth the size of Giardia.
With the germs that cause
gastrointestinal distress, E. coli,
Shigella, Giardia, Cryptosporidium and
Norovirus, swallowing between 10 and
100 of these pathogens can cause an
individual to become sick. It takes 1,000
to 10,000 cells to cause Pseudomonas
or Legionella illnesses, but even with
these germs, the water will appear clear.
Relying on the appearance of the water
is not a dependable or safe method for
management.
What should operators be doing
if simply looking at the water is not
adequate? Regular, repeated water
testing is the most important safety
precaution that pool operators can
take to prevent RWIs. To ensure there
is enough free chlorine, a DPD test kit
must be used. When DPD or FAS-DPD
(a type of DPD) kits are used, any
chlorine present will make the sample
turn pink/red. There is another type of
test kit (OTO) where the water turns
yellow with chlorine. OTO kits are not
acceptable for public pools and have
not been considered reliable for public
health monitoring since 1975. Many
different brands of test strips are readily
available and will provide some estimate of the free chlorine concentration, but
test strips are not precise enough for
public pools and must be avoided.
To maintain sanitized water and
prevent RWIs, certain requirements
must be met. At least 1 ppm of free
chlorine, a pH between 7.2 to 7.8, and
a cyanuric acid (CYA) level of 100 ppm
or lower. The free chlorine, pH and CYA
must be checked routinely and adjusted
as needed. If either free chlorine or
pH is out of range, the pool must be
closed for use until the concentration(s)
are within range. The frequency of
testing should be sufficient to ensure
that free chlorine, pH and CYA remain
within range, and adherence to local
pool code requirements is essential,
especially for public pools. Additionally,
water balance parameters, such as total
alkalinity, calcium hardness and TDS, are
important. They may impact disinfection
if they allow pH to fluctuate out of range.
It is safe to swim when they are out of
range as long as the free chlorine and
pH are in range, as they are the two
most critical parameters. Water balance
can be checked less frequently than free
chlorine and pH.
How much chlorine is the correct
amount to prevent RWIs? Unfortunately,
there is no easy answer to this question.
For lightly used pools like residential
ones and many hotel, apartment,
condominium and HOA pools, 1 ppm
for pools not using cyanuric acid, or
2 ppm when cyanuric acid is used, is
normally all that is required. However,
once the bather load increases, such
as with a pool party or swim meet, the
minimum concentration before the
event starts must be higher. This is
because a large influx of swimmers can
and will readily deplete the free chlorine
concentration. The perspiration, urine,
sunscreens and skin surface of the
swimmers all are organic compounds
that react with free chlorine. Once the
free chlorine concentration drops, any
fecal germs shed by the swimmers or
Pseudomonas cells liberated from the
traces of biofilm are suddenly available
to possibly induce illness.
A free chlorine concentration of 1
ppm for pools not using cyanuric acid or
when cyanuric acid is less than 50 ppm,
or 2 ppm when cyanuric acid is used at
50 ppm or more, is the minimum that
must be maintained at all times and in
all areas of these bodies of water. This
must not be interpreted as the desired
target for normal operation. The best
practice to prevent RWIs is to establish
a higher concentration overnight so
that it is available to oxidize the organic
load introduced by the swimmers. This
is because many feed systems are not
able to supply substantial amounts of
chlorine during rapid increases in bather
loads. The maximum concentration
of free chlorine must comply with the
product label directions. Swimmers
may reenter the body water when the
free chlorine concentration falls below
4 ppm. For knowledge of industryaccepted
standards, refer to ANSI/
APSP/ICC-11 2019 American National
Standard for Water Quality in Public
Pools and Spas.
The 1 ppm minimum concentration
of free chlorine is well supported in U.S.
pools from two different viewpoints.
Scientific literature has reported
numerous outbreaks of E. coli, Shigella,
Pseudomonas, Giardia and Norovirus
during the past 50 years. But there is
not a single documented case of any
one of these germs causing illness
when at least 1.0 ppm of free chlorine
was measured in the water. A separate
study from Australia also concluded
that these chlorine-sensitive germs will
be adequately controlled by 1.0 ppm of
free chlorine. The one exception to this
is Cryptosporidium. Crypto cannot be
controlled by normal concentrations
of free chlorine, with or without the
presence of cyanuric acid. If Crypto is a
concern, such as in wading and therapy
pools, an adequately sized Secondary
Disinfection System described in ANSI/
APSP/ICC-11 2019 American National
Standard for Water Quality in Public
Pools and Spas must be installed.
It is not uncommon to encounter
stories describing algae as sources of
RWIs. There are no documented cases
of illnesses from the types of algae
that grow in swimming pools. Some
individuals claim that algae can harbor
RWI-causing germs such as E. coli.
While this is theoretically possible, it
has never been documented. If the
water is turbid and green, the probable
cause is “green algae.” Green algae are
easily controlled by low concentrations
of free chlorine, so where it is found,
there is a high likelihood that there is
less than 1.0 ppm of free chlorine in
the water. Mustard and black algae are
considerably more difficult to control
due to their physical structure as well as their stationary growth pattern
on the pool surface. To control both
mustard and black algae, it is essential
that the area be rigorously brushed
and the concentration of free chlorine
be elevated. The brushing disturbs the
algal colony and helps to increase the
contact of the cells with the chlorine. It
also removes the surface layer of growth
and permits the chlorine to contact the
deeper buried algal cells still attached to
the surface.
Are there any occasions where
visual appearance alone confirms the
lack of free chlorine? Yes, but they are
limited. The first is when the water is
dull or slightly cloudy. In this case, the
root cause could be low chlorine or a
filtration and circulation issue. Both
must be addressed. The other is with
green algae. When the water is turbid
and green with algae, it is a safe bet that
there is less than 1 ppm of free chlorine.
Other than these two occasions, testing
is the only way to ensure that adequate
disinfection is present to control RWIs.
Ultimately, a pool operator’s goal
is to prevent RWIs by establishing
and maintaining proper disinfection
measures. Diligent, repeated water
testing is vitally necessary, and a
conscientious operator will never rely
solely on visual cues to confirm water
safety
REFERENCES
1. Estimating Waterborne Infectious Disease
Burden by Exposure Route, United States,
2014. Emerging Infectious Diseases, Volume
29, Number 7 – July 2023.
