Trichloramine prevention
remains better than cure
Recreation is the first UK publication to feature new
research from Germany that is relevant to anyone involved
in managing, working in or using indoor swimming pools
This German research, published here for
the first time in the UK, provides an
important step forward in our
understanding of the relationship between
chlorine levels in pools and the production
of trichloramines. Essentially, high
combined chlorine levels do not
automatically mean high trichloramine
The key precursor for the formation of
trichloramines (nitrogen trichloride) is urea
from urine, sweat and skin cells. Best
practice remains largely unchanged: that
the concentration of urea in pool water
must be minimised.
Keys to minimising urea concentration:
● Educate pool users: prevention is better
than cure. Comprehensive pre-swim
hygiene measures including using the
toilet (elimination of the urine source)
and washing thoroughly prior to pool use
(the skin source) will help greatly.
● Remove urea by water treatment through
ozone-activated carbon treatment or
● Reduce the urea concentration by dilution
(adding 30 litres of fresh water per pool user).
● Provide good pool hall ventilation, ideally
without re-circulation or at least 30 per
cent fresh air.
solubility, it readily escapes from swimming and
bathing pool water and may consequently accumulate
in the air of indoor pools and then lead to breathing
problems and eye irritations. The irritating effects
are similar to those of chlorine gas [2].
Belgian researchers hypothesised that the
exposure of schoolchildren to trichloramine during
visits to indoor chlorinated swimming pools
adversely affected the lung epithelium permeability
of the children and could lead to an increased risk
of developing asthma [3].English scientists reported
asthma symptoms in lifeguards and swimming
teachers caused by chloramines [4].More recent
studies corroborate the above hypothesis [5, 6].
In summer 1999, the German Federal
Environmental Agency, as a precaution, started to
measure trichloramine in the air of indoor pools as
part of scientific investigations into the formation
and minimisation of undesirable by-products of
swimming and bathing pool water chlorination.
The object was to obtain initial information as to
whether and to what extent the air in German
indoor swimming pools is contaminated by this
compound.No such information was available for
German indoor pools at the time.
The following is a report on the formation and
properties of trichloramine and its analysis. First
measurement results are presented and discussed.
2. Urea and formation of trichloramine in
pool water
Considerable amounts of urea are introduced to
swimming and bathing pool water by pool users.
The sources are the skin, urine and sweat.Urea is
the main final product of the protein metabolism
of humans. About 90 per cent is excreted via the
kidneys (urine), the remainder via sweat and
intestinal secretions. It also forms during skin
Urea is a chemical compound with the following
formula:H2N-CO-NH2. In its pure form, it forms
colourless and odourless crystals which are readily
soluble in water. The presence of urea in
chlorinated pool water leads to the formation of
Urea sources: skin, urine and sweat
The skin is the largest organ of the human body,
with a surface area of approximately 1.5 to 2m2.
Urea is a product of the degradation of the amino
acid arginine during skin hornification [7]. It
belongs to the natural factors that keep the skin
The urea content in the horny layer (stratum
corneum) of healthy skin is about 8 μg per cm2 of
skin surface, for both men and women. 2m2 of
skin surface would thus contain about 0.16 g of
urea. Pool water readily removes water-soluble
organic and inorganic constituents including urea
from the skin of pool users.Assuming that all urea
in the stratum corneum is fully washed into pool
water in this way, then 1,000 pool users would
release about 160g of urea into pool water.
Thorough washing and showering by pool users
prior to pool use removes about 75 to 97 per cent
of the urea contained in the stratum corneum and
is thus a very effective way to prevent urea input
into pool water (Figure 1). Substantial amounts
of urea and other nitrogen compounds may also
be introduced into pool water through urine and
sweat.Table 1 lists average concentrations of urea
in urine, sweat and the horny layer of the
Different figures are given in the literature as
regards urine and sweat input to pool water [9 –
13].Assuming a urine input of 35 ml per pool user
as determined by Gunkel and Jessen [9], the input
of urea to pool water would be about 0.8g per pool
user. The amount of sweat released to pool water
per pool user depends on many factors, such as
water temperature, air humidity, physical
condition and activity of the pool user. The expert
literature indicates that an active swimmer, for
example, may excrete up to one litre of sweat per
hour [14].Urea input with one litre of sweat would
amount to about 1.5g per pool user and hour.
Trichloramine formation mechanism
In the scientific literature, the mechanism of
trichloramine formation from urea is discussed
from three different directions:
• Enzymatic degradation of urea, by the enzyme
urease which is contained in various bacteria, to
ammonia or ammonium, and reaction of the latter
with free chlorine to trichloramine.According to
Jessen and Gunkel [13], this process does not occur
in chlorinated pool water;
• Hydrolysis (cleavage by the action of water) of
urea,with formation of ammonia or ammonium,
and subsequent reaction with free chlorine to
trichloramine. This occurs only at temperatures
of more than 65C and is not, therefore, relevant to
pool water;
• The decisive mechanism for the formation of
trichloramine in pool water is the step-by-step
March 2006 recreation ● 31
Figure 1: Influence of washing on the urea content in the stratum corneum, after (8).
Table 1: Average concentration of urea in urine,
sweat and the horny layer of the epidermis.
Shower gel + water
Urea content in skin after washing
35% Test person A
Test person B
Test person C
Test person D
8 μg/cm_
Table 3: Henry’s law constants (H).
Hypochloric acid 0.069
Monochloramine 0.45
Dichloramine 1.52
Trichloramine 435
1) Eichelsdörfer et al. [16] ; 2) Spon [17]
Table 2: Properties of chlorine and chloramines.
Free chlorine Mono Dichloramine Trichloramine
No eye irritation in rabbits1) 0-8 0-2 No data No data
Distinct eye irritation 30 4 No data No data
in rabbits1)
Odour and taste threshold2) 20 5 0.8 0.02
O = C + 2 HOCl 2 NCl3 + CO2 + H2O
1,1,3,3- hypochloric acid trichloramine
tetrachlorourea ➞ ➞
reaction of the urea introduced by pool users with
free chlorine to 1,1,3,3-tetrachlorourea and finally
to trichloramine, as described in the literature [15].
Properties of trichloramine
Trichloramine is an undesirable by-product of
disinfection,which has a strong irritating effect on
the eyes, nose, throat and bronchial tubes. Its
odour is similar to that of chlorine. The odour and
taste threshold in water is very low, at 0.02 mg/l.
A threshold concentration for eye irritation caused
by the presence of trichloramine in pool water has
not been established to date.
Eichelsdörfer et al. [16] demonstrated for free
chlorine and monochloramine that distinct eye
irritation in rabbits does not start at a
concentration lower than about 30 mg/l and 4
mg/l, respectively. The value for trichloramine
ought to be markedly lower. Table 2 summarises
the literature data.
Previously, it had long been assumed that
trichloramine forms only at a pH less than or equal
to 4.4. However, this view has had to be revised;
trichloramine is also formed at higher pH values
such as occur in pool water, and is rather stable
under such conditions. Investigations have shown,
for example, that a diluted aqueous trichloramine
solution has a half-life of 218 minutes at a pH of
7. This means that 50 per cent of the substance
decomposes in water during that time [18]. For
example, when the trichloramine concentration
in pool water is 0.1 mg/l, then it would be 0.05
mg/l after 218 minutes, if one ignores gaseous
emissions of the compound to air.
The outgassing behaviour of a substance
dissolved in pool water can be estimated using the
air/water partition coefficient (= Henry’s law
constant, H). The lower the Henry’s law constant,
the more soluble is the substance in pool water.
The higher its Henry’s law constant, the more
readily it escapes from pool water to air. The
Henry’s law constants of mono-, di- and
trichloramine and hypochloric acid have been
determined experimentally by Holzwarth et al.
[19] (Table 3).
The H values show that trichloramine escapes
from pool water 966 times faster than
monochloramine and 286 times faster than
dichloramine. It ‘feels’ 435 times ‘more
comfortable’ in indoor pool air than in pool water.
This and its odour and taste threshold (Table 2)
are the main reasons for the typical chlorine-like
smell in swimming pool halls. The H value of
dichloramine is only of theoretical interest, as the
compound is not stable and decomposes very
quickly in pool water [20].
Water attractions such as waterslides, water
geysers, flood showers and water fountains
accelerate the release of trichloramine to air.
Comparing, for example, the outgassing behaviour
of trichloramine to that of chloroform, which
belongs to the substance group of
trihalomethanes, trichloramine escapes from pool
water three times faster than that substance.
3.1 Measurement in pool water
There is currently no simple on-site method for
the selective determination of trichloramine in
pool water. One laboratory method to reliably
differentiate between and quantify the various
inorganic chloramines – mono-, di- and
trichloramine – is membrane introduction mass
spectrometry (MIMS), whose use remains
reserved to specialised water analysis laboratories.
The detection limit for trichloramine is reported
to be 0.06 mg/l [21].
4.2 Measurement in the air of indoor
swimming pools
The German Federal Environmental Agency,
Department for Drinking and Swimming Pool
Water Hygiene, began measuring trichloramine
in the air of indoor swimming pools in summer
1999, for precautionary and the following other
reasons: no data whatsoever was available on
trichloramine concentrations in the air of German
swimming pool halls; the French INRS (Institut
National de Recherche et de Sécurité) has
published a validated method for the
determination of trichloramine in air [22], which
was adopted by the Federal Environmental Agency
to ensure comparability with INRS measurement
data and which is, to this day, the only existing
method for determination of trichloramine in air;
a health-based guideline value of ≤ 0.50 mg/m3
has been proposed in France for trichloramine in
indoor pool air [2, 22]. This value can be used as
a basis for assessment of the measurement results.
The principle of the analytical method is shown
in the flow chart, left.
5. Selected results
Table 4 presents selected measurement results on
trichloramine in the air of indoor swimming pools
for different pool types and compares them with
32 ● recreation March 2006
Table 5: Influence of air renewal on trichloramine concentration in indoor pool air
Flow chart: the principle of the analytical method.
Contribution of fresh air to Trichloramine Chloramines (expressed as
ingoing air mass flow in air combined chlorine) in pool water
% mg/m3 mg/l
0 0.52 0.15
30 0.37 0.15
1) according to INRS [22] ; 2) according to the German standard DIN 19643-1 [23]
Table 4: Trichloramine concentrations in the air of indoor swimming pools and corresponding
concentrations of combined chlorine in pool water.
Pool type Trichloramine Chloramines (as combined chlorine)
in indoor pool air in pool water
mg/m3 mg/l
Leisure 0.13 0.07
Leisure 0.16 0.13
Leisure 0.37 0.80
Leisure 2.2 0.12
Conventional 18.8 0.25
Hydrotherapy 0.19 0.01
Hydrotherapy 0.14 0.05
Exercise pool 0.05 0.03
Guideline values 0.501) 0.202)
Chloride concentration
per sample volume
➞NCl3 concentration
per m3 of air
➞ ➞ ➞ ➞
NCl3, NH2Cl,
HOCl, etc.)
isolation of NCl3 by
trapping soluble
chlorine compounds
in an adsorption tube
Concentration of
NCl3 and chemical
transformation to
chloride in a special
treated filter cassette
of chloride and
by ion
‘Water attractions such as
waterslides, water geysers,
flood showers and fountains
accelerate the release of
trichloramine to the air’
the corresponding measurement data for
combined chlorine in pool water.
The values in Table 4 show that measured
trichloramine concentrations in the air of indoor
swimming pools do not correlate with the values
for combined chlorine. There may be cases where
the concentration of combined chlorine in pool
water, at 0.80 mg/l, exceeds by far the upper value
of 0.20 mg/l recommended by the German
standard DIN 19643-1 while the trichloramine
concentration in the indoor pool air, at 0.37
mg/m3, is below the recommended guideline value
of 0.50 mg/m3.
Conversely, there are cases where concentrations
of combined chlorine in pool water comply with
(0.12 mg/l) or are just slightly above (0.25 mg/l)
the recommended upper value while the
corresponding results for the trichloramine
concentration in indoor pool air (2.2 and 18.8
mg/m3) exceed the guideline value, in the one case,
by a substantial amount. This means that a DINcompliant
concentration of combined chlorine in
pool water is not automatically linked with
trichloramine concentrations in the indoor pool
air that are safe for human health.
For this reason, care must be taken to ensure
that the ventilation system is designed so that
during pool operating hours the proportion of
fresh air fed to the air circulating in the pool hall
is adjusted to the pool capacity utilisation rate, as
prescribed by the technical rule VDI 2089-1 [24].
When the pool is used to maximum capacity (for
example,with very high bather loads and with all
water attractions switched on) the proportion of
fresh air should be at least 30 per cent of the
ingoing air mass flow.An example of the influence
of air renewal via the contribution of fresh air to
the ingoing air mass flow is presented in Table 5.
While the upper value in DIN 19643-1 for
combined chlorine in pool water is complied with,
at 0.15 mg/l, trichloramine can build up in the
indoor pool air to exceed the guideline value of
0.50 mg/m3 if there is no air renewal by a defined
proportion of fresh air (no dilution effect).
6. Discussion and conclusions
No direct correlation exists between the
trichloramine concentration in the indoor pool
air and the corresponding value for the chemical
parameter ‘combined chlorine’ in pool water.This
is because the measurement result for this sum
parameter does not, unfortunately, indicate how
much of the total content is trichloramine. A
simple and reliable on-site method for specific
measurement of trichloramine as an individual
substance in pool water does not yet exist.
A DIN-compliant concentration of combined
chlorine in pool water is no automatic guarantee
that the trichloramine concentration in the air of
the indoor pool will be tolerable from a health
perspective. In addition, this means that, during
pool operating hours, the airborne trichloramine
should be diluted by air renewal via a defined
contribution of fresh air to the ingoing air mass
flow in accordance with the generally accepted
technical standards (VDI 2089 Blatt 1) [24]. This
will prevent trichloramine accumulating in the air
of the pool hall to an extent as to exceed the
guideline value of 0.50 mg/m3.
The concentration of urea in pool water must
be minimised, since its reaction with free chlorine
in pool water results in the formation of
trichloramine, among other substances. This may
be achieved by the following:
• With the help of pool users: by using the toilet
(elimination of the urea source ‘urine’) and
washing themselves thoroughly (elimination of
the urea source ‘skin) prior to pool use;
• Removal of urea by water treatment (e.g. ozoneactivated
carbon treatment [25] [26], photooxidation
[27]); and
• Reducing the urea concentration by dilution
(adding 30 litres of fresh water per pool user).
If these hints are observed, there need be no
concern, according to present knowledge, that
trichloramine in indoor swimming pool air may
pose a health risk.
March 2006 recreation ● 33
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16 September 2003.
Dr Ernst Stottmeister and
K Voigt are from the
German Federal
Environmental Agency.


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