Paper #482
Ozone interactions with human hair: ozone uptake
rates and product formation
Presented at the 2007 Air and Waste Management Association Annual Conference and
Exhibition
Lakshmi Pandrangi, Glenn Morrison
University of Missouri-Rolla, 1870 Miner Circle, Rolla, MO 65409
ABSTRACT
In laboratory experiments we quantify the ozone uptake rate for human hair and
the resulting formation of volatile aldehydes and ketones. Hair samples were exposed to
ozone in a tubular Teflon reactor. The integrated ozone uptake was quantified by
measuring the inlet and outlet ozone concentration over a 24 hour period. Volatile
reaction products were identified and quantified by sampling the reactor inlet and outlet
using Tenax sorption tubes followed by thermal desorption and GCMS analysis. Volatile
reaction products consistently observed in all experiments include geranyl acetone and 6methyl 5-hepten 2-one. These are products of the ozone – squalene reaction. Compounds
formed as a result of ozone chemistry with personal care products have yet to be
identified. On average, the integrated ozone uptake for the washed hair samples was
1.12×10-5 mol O3 g-1 and for unwashed hair was 1.87 ×10-5 mol O3 g-1. For a hair sample
that was not washed for 3 days ozone uptake was about ten times higher than the average
for all other hair samples. Further the yield of geranyl acetone, 6 methyl 5-hepten-2-one
and decanal was consistently higher on unwashed hair. While natural oils that coat hair
contribute to ozone reactions on unwashed hair, ozone appears to be reacting with other,
unidentified compounds on washed hair.
INTRODUCTION
Reactions between ozone, hair oils and personal care products used on hair can generate a
variety of volatile organic compounds (VOCs). These reactions may reduce ozone
1
concentrations but increase reaction product concentrations in the breathing zone.
Weschler et al. (2005)1 conducted experiments in a simulated aircraft experiment with
soiled t-shirts having human skin oils and showed that there are reactions taking place
between ozone and squalene that is present in the skin oils or sebum. Sebum is composed
of about 12% squalene, 57.5% triglycerides and free fatty acids. Sebum found on the skin
surface has 30% free fatty acids due to hydrolysis of a portion of triglycerides by the
resident bacteria in the ducts.2 Ozone can react rapidly with the double bonds present in
any of these compounds. Shown in Figure 1 are several possible reaction pathways when
ozone reacts with squalene. Reaction products include geranyl acetone and 6-methyl-5hepten-2-one. These compounds are unique to the ozone-squalene reaction and serve as
signature compounds for this reaction. Studies show that ozone reacts with skin lipid
containing squalene to form acetone, 4-oxopentanal, nonanal and decanal apart from the
above mentioned two compounds.3
Use of personal care products on hair may also influence ozone uptake and product
formation on hair. Personal care products used on hair, such as soaps and conditioners,
often contain reactive fatty acids and terpenes. Volatile products from the ozone-fatty
acid reaction include straight-chain aldehydes and acids. For example, the reaction of
ozone with oleic acid (or its ester in a triglyceride) can generate nonanal, nonanoic acid,
and several other decomposition products. Ozone reactions with terpenes can generate
numerous products, some of which are sensitizers or carcinogens (e.g. formaldehyde).
Figure 1 Squalene-ozone reactions and products
2
O
acetone
O3
O
6-methyl-5-heptenone
O
6,10-dimethyl-5,9undecadienone
squalene
carboxylic acids and other products
The human scalp secretes a large quantity of sebum to coat hair, but hair is frequently
washed or processed with other chemicals. Given the proximity to the breathing zone and
the unknown character and extent of these reactions, we have studied ozone uptake and
volatile reaction product generation from washed and unwashed hair.
OBJECTIVES OF PRESENT STUDY
The main objectives of this study are to:
1. Quantify the integrated ozone uptake on human hair.
2. Identify and quantify the volatile product emission rates formed as a result of
ozone chemistry.
3. Quantify the ozone-specific product yield.
4. Evaluate the influence of ethnicity and washing by comparing ozone uptake and
product yields on washed and unwashed hair from several ethnic groups.
MATERIALS AND METHODS
Hair Sample Collection
Washed and unwashed hair samples were collected from people belonging to Chinese,
African American, Caucasian, Indian (Asian) ethnic groups. Washed hair samples were
collected on the day the hair was washed. Unwashed hair samples were collected after the
hair is left unwashed for two to three days. The hair samples thus obtained were stored in
foil packets until the experiment was conducted
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Experimental Methods
Apparatus
The apparatus showed in Figure 2 was used to conduct the experiments.
Figure 2 Apparatus used for experiments to pass ozone through the hair samples,
measure the ozone uptake rate and collect volatile product samples. M1, M2, M3 are
mass flow controllers.
Exhaust
Bypassed during
experiment
1000 sccm
M1
Humidifiers
1000 sccm
Bypassing the Reactor
M2
50% RH +
Ozone
M3
Ozone
Generator
Air with ozone
Passing through the reactor
PFR
5 sccm
O3
Analyzer
Dry
Compressed
Air
Hair
Bypassed during
calibration
By use of humidifiers and an ozone generator, ozone containing air, at 50% relative
humidity (RH), was delivered to a reactor containing hair samples. Dry compressed air is
passed through three mass flow controllers M1, M2 and M3 maintained at 1000sccm,
1000sccm and 5sccm respectively. Air from M2 is passed through humidifiers. Air from
M3 is passed through an ozone generator. Air from M1, M2 and M3 are combined,
resulting in a 50% RH stream containing ozone. The typical reactor inlet ozone mixing
ratio was about 120 ppb. This stream is delivered to the flow-through reactor. The reactor
used in these experiments is a ¾ inch diameter and 4.5 long Teflon tube. Inlet and outlet
4
air are sampled by an ozone analyzer to obtain the concentration of ozone. Samples of
inlet and outlet air are collected to analyze the volatile organic compounds (VOCs)
formed. The ozone concentration is recorded on a computer that is connected to the
ozone analyzer.
Experimental Procedure and VOC Sample Collection
The reactor was cleaned with methanol before each experiment. Initially, two air samples
are collected at the outlet: the first with humidified air passing through the empty reactor
and the second with humidified air containing ozone passing through the reactor. These
samples are acquired after passing air and ozone for 10 minutes through the reactor. All
air samples are collected on Tenax-packed sorption tubes (a stainless steel tube packed
with a 2, 6-diphenylene-oxide polymer resin polymer resin) using an external sampling
pump. The Tenax tubes are analyzed with a Thermal Desorption GC-MS (TD/GC-MS)
for identifying and quantifying the VOCs formed during the experiment. The reactor is
then packed with hair. The hair is weighed after the experiment. Any portion of the
reactor not filled with hair is packed with cleaned glass wool. Again, humidified air is
passed through the reactor and a VOC sample is collected to obtain a background reading
on the hair without ozone. Ozone is then introduced and set to bypass the reactor until the
ozone concentration stabilizes. Ozone and VOC concentrations in this bypass air
represent the initial inlet concentrations of the reactor. At this point, the stream is
redirected to the reactor. An air sample is collected at a point where the ozone
concentration starts to increase. This sample is analyzed to identify and quantify VOCs
formed. During the experiment, the outlet ozone concentration rises as reactive surface
compounds become consumed. Eventually, the concentration becomes relatively constant
(after about 24 hours of exposure). At this time, the flow is again redirected to bypass the
reactor and the inlet ozone concentration is recorded. All ozone concentrations are
monitored and recorded with a computer. Because the inlet concentration is only
recorded at the beginning and end of the experiment, we assume that the inlet
concentration varies linearly in time from beginning to end. The area between the inlet
and outlet ozone concentration curves is integrated using MATLAB to quantify the
integrated ozone uptake by hair during the course of the experiment. Figure 3 shows a
typical plot of the inlet and outlet ozone concentrations. The shaded area between inlet
5
and outlet concentrations is proportional to the amount of ozone consumed during the
experiment.
Figure 3 Graph showing inlet and outlet ozone concentrations. Shaded area between the
curves represents the amount of ozone consumed.
140
Inlet Concentration
Conc(ppb)
120
100
80
Outlet Concentration
60
40
20
0
0
2
4
6
8
10
12
14
16
Time(hrs)
EQUATIONS AND CALCULATIONS
The concentration of outlet ozone is plotted against time in hours. Inlet ozone
concentration is calculated as a linear function of time and plotted on the same graph as
outlet concentration. The area between the two curves is calculated using MATLAB. The
area thus obtained is in units of ppb-hr. The integrated ozone uptake is then calculated as
shown below
Equation 1.
M = Area × Q
where:
M = Mass of ozone that reacted with hair, ppb-m3
Area = Area between inlet and outlet concentration curves, ppb-h
Q = Airflow rate, m3/h
Equation 2.
6
1000 × Cppm × M03
V
n
where :
C=
C = ozone concentration in μg/m3
Cppm = ozone concentration in ppm
RT
V/n =
P
P = Atmospheric pressure, atm
V = Volume of gas, liters
n = number of moles of ozone
R = Gas constant = 0.082057 liter atm mol-1K-1
MO3= Molecular weight of ozone, g/mol
Equation 3.
Cppb =
C×V
n
M O3
where:
Cppm = ozone concentration in ppm
C = ozone concentration in μg/m3
V = Volume of gas, liters
n = number of moles of ozone
MO3= Molecular weight of ozone, g/mol
Equation 4.
Mass of ozone reacted (ppb - m 3 ) × M 03 (g/mol)
Mass of ozone reacted (μg) =
V
(liter/mol)
n
Equation 5.
Moles of ozone reacted =
Mass of ozone reacted (g)
M 03 (g/mol)
Equation 6.
7
Integrated ozone uptake (mol O3. g-1) =
Moles of ozone reacted (moles)
Weight of sample (g)
Integrated ozone uptake is calculated in the units mol O3 g-1 using equation 7.
RESULTS AND DISCUSSION
Characteristics of Hair Samples
Details of ethnicity, shampoos or conditioners used on hair and the number of days hair
was unwashed are shown in Table 1. None of the samples are from smokers.
Table 1 Characteristics of the hair samples
Sample
Ethnicity
Washed
H1A
Chinese
×
H1 B
Chinese
H2 A
African
American
H2 B
African
American
H3 A
Caucasian
H3 B
Caucasian
H4 A
Caucasian
H4 B
Caucasian
H5 A
Caucasian
H5 B
Caucasian
H6 A
Indian
Subcontinent
H6 B
Indian
Subcontinent
Unwashed
Notes
Hair shampooed and conditioned with
Garnier Frutics shampoo and conditioner
×
Hair was not washed for 2 days
Hair washed with Head and Shoulders
shampoo
×
×
Hair not washed for 1 day
Hair washed with Suave shampoo and
conditioner
×
×
Hair was not washed for 1 day
Washed with Suave natural shampooOcean Breeze
×
×
Hair not washed for 3 days
Hair washed with Matrix Biolage shampoo
and conditioner
×
×
Hair not washed for 1 day
Hair was permed week before experiment.
Washed with Curls Rock shampoo and
conditioner. Applied Curls Rock gel.
×
×
8
Hair not washed for 2 days
H7 A
Indian
Subcontinent
H7 B
Indian
Subcontinent
H8 A
Indian
Subcontinent
H8 B
Indian
Subcontinent
Hair washed with Sunsilk shampoo and
applied L’Oreal conditioner.
×
×
Hair not washed for 2 days
Hair washed with Head and Shoulders
shampoo
×
×
Hair not washed for 3 days
Integrated Ozone Uptake
Integrated ozone uptake rate was calculated and reported as reactivity in moles of ozone
per gram of hair (mol O3 g-1). Samples H1 and H2 are of Chinese and African American
respectively; H3, H4 and H5 are of Caucasian origin; H6, H7 and H8 are of Indian
subcontinent. Figure 4 shows the results obtained for different hair samples.
Figure 4: Ozone reactivity with human hair (moles of ozone per gram of hair)
Washed
Reactivity of ozone with human hair
Reactivity of hair with ozone
(moles/gm hair)
Unwashed
7.E-05
6.E-05
5.E-05
4.E-05
3.E-05
2.E-05
1.E-05
0.E+00
1
2
3
4
5
6
7
8
Sample Name
For samples H1, H3, H6 and H7 washed hair had a somewhat higher reactivity than
unwashed hair. These hair samples had been treated with conditioners. Application of
conditioners could have resulted in increase of ozone reactivity. In addition to the above
mentioned samples H5 was also treated with a conditioner. However washed hair sample
of H5 has lower reactivity compared to unwashed hair sample. This could be due to
individual differences, such as more sebum secreted by individual H5. More sebum will
result in more squalene available for ozone reactions. Samples H2, H4, H5 and H8 have a
higher reactivity for unwashed hair samples. Among these H2, H4 and H8 were only
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shampooed with no other treatments. Another interesting observation is that all Caucasian
samples (H3, H4 and H5) show similar reactivity for washed hair. Individuals H2, H4
and H8 have shorter hair compared to the rest of the individuals. Unwashed Samples H2,
H4 and H8 have much higher reactivity than washed samples. The reason for this could
be that the hair samples collected from individuals having shorter hair will be closer to
the scalp and hence gets coated with more reactive sebum.
Average reactivity of washed hair samples and unwashed hair samples are 1.12×10-5 mol
O3 g-1 and 1.87 ×10-5 mol O3 g-1 respectively. The average reactivity of unwashed hair
sample is 1.67 times the average reactivity of washed hair. The highest reactivity is about
100 times the lowest reactivity (low = 5.14×10-7 mol O3 g-1 and high = 6.20×10-5 mol O3
g-1).
Secondary Emissions Rates
For most of the hair samples the secondary emission rates for formaldehyde,
acetaldehyde and acetone could not be obtained with accuracy as the primary emission
rates for these compounds were relatively high. An example plot is shown in Figure 5.
The figure shows that the emission rates of acetaldehyde and acetone are much higher, on
a molar basis, than most other compounds. In general, these high rates made the
determination of secondary emission rates highly uncertain; thus, they are not reported
for C1-C4 or acetone.
Figure 5: Primary and secondary emission rates
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Blank
4.22
9.47
3.49
4.02
w/o ozone
w ozone
C
4
C
3
A
C
2
C
1
G
M
E
A
H
ex
an
al
N
on
an
al
D
ec
an
al
0.2
0.18
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
H
nmoles/g-s
2.45
Compounds
Yield
The “yield” is an indicator of chemical mechanisms on the surface. In this case, Yield is
defined as moles of product formed per mole of ozone that reacts. Compounds that were
consistently observed in most of the experiments are 6-methyl 5-heten 2-one and geranyl
acetone. Yield for these compounds has been calculated and reported in Figure 5 and
Figure 6 respectively.
Figure 6: Yield for 6-methyl 5-heten 2-one
Unwashed
Washed
MEH
Yield
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
H1
H2
H3
H4
H5
Sample
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H6
H7
H8
Figure 7: Yield for geranyl acetone
Unwashed
Geranyl Acetone
Yield
Washed
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
H1
H2
H3
H4
H5
H6
H7
H8
Sample
Figure 8: Yield for decanal
Unwashed
Decanal Yield
Washed
0.50
0.45
0.40
Yield
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
H1
H2
H3
H4
H5
H6
H7
H8
Sample
In Figure 6 all the samples except H1 and H7 show a higher yield for unwashed hair
samples. From Figure 7 it is evident that except for H3 all the unwashed samples have
higher yields. 6-methyl 5-heten 2-one and geranyl acetone are products formed from
reaction between ozone and squalene. In Figure 8 all the samples except H4 and H7 show
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higher yield for decanal. In this case, the higher yield for unwashed hair samples indicate
that ozone is reacting with the fatty acids instead of squalene.
A comparison of yields with reactivity suggests that 1) on unwashed hair ozone reactivity
is primarily due to reactions with sebum compounds and 2) on washed hair, ozone
reactivity is due to reactions with something other than sebum. Unwashed hair
consistently reacts to generate higher yields of ozone-squalene products. However, ozone
reactivity is not consistently higher on unwashed hair. Therefore, ozone must be reacting
with other compounds on the washed hair. Since soaps, conditioners and shampoos are
composed of fatty acids, some unsaturated, ozone may be reacting with these compounds
that remain after treatment.
CONCLUSIONS
Experiments conducted show that the average reactivity of unwashed hair samples is
higher than the average reactivity of washed hair. However, only four of eight samples
had higher ozone reactivity for unwashed hair. The use of personal care products may
strip hair of natural oils and form a less-reactive surface resulting in reduced reactivity
with ozone. However, washing might also coat hair with ozone-reactive compounds.
Experimental results show that most of the washed hair samples that were not treated
with conditioners or gels have lower reactivity than those which were treated with any of
the above mentioned products. Hair samples of Caucasian origin demonstrated little
difference in ozone reactivity for washed or unwashed samples. Hair from individuals
having shorter hair was more reactive than from individuals having longer hair.
Unwashed hair has a higher yield for 6-methyl 5-heten 2-one and geranyl acetone
compared to washed hair samples. From this result it can be inferred that squalene
present in hair oils is reacting with ozone. Most unwashed hair samples had a higher yield
for decanal than washed hair, indicating that ozone is reacting with the fatty acids. There
is no particular trend observed in the yield of hexanal, nonanal, formaldehyde,
acetaldehyde, acetone, propanal and butanal. Ozone reactivity, and the yield of the
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compounds formed, mostly depends on whether hair is washed or unwashed rather than
on ethnicity.
REFERENCES
1. Armin Wisthaler , Gyöngyi Tamás , David P Wyon , Peter Strøm-Tejsen , David
Space , Jonathan Beauchamp , Armin Hansel , Tilmann D Märk , Charles J
Weschler. Environ Sci Technol. 2005, 39, pg. 4823-4832
2. Pierre (EDT) Agache, Philippe (EDT) Humbert. “Measuring the Skin: Noninvasive Investigations, Physiology, normal Constants.” Springer-Verlag Berlin
Heidelberg publication, New York, 2004, ISBN 3-540-01771-2, pg. 271
3. Fruekilde. P, Hjorth. J, Jensen. N. R, Kotzias. D, Larsen. B. Atmospheric
environment. 1998, 32, pg. 1893-1902
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