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Factors affecting the strength of hair

Factors affecting the strength of hair

Introduction: (Initial Observation)

Hair pulled by brush, comb and hair band may break if it does not have enough strength. Longer hairs can be pulled by seat belts, chairs or many other objects around us.

If the tension on hair exceeds the tensile strength of the hair, it will break. That is why you may find many broken hair on your brush and around your room.

Physical, environmental and chemical factors can affect the strength of the hair. Some may increase the strength and some may reduce the strength. Some may have a temporary effect and others may have a permanent effect. For example the effect of moisture and water is temporary but chemical and mechanical damages may be permanent.

In this project we investigate to see how do different environmental factors, water and chemicals affect the strength of the hair.


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.  

Project advisor

Information Gathering:

Find out about what you want to investigate. Read books, magazines or ask professionals who might know in order to learn about the effect or area of study. Keep track of where you got your information from.

Learn about the hair structure and biology. Determine how do you want to test the tensile strength of the hair? How do you want to test the effect of different factors?

Search the Internet for keywords like “hair strength”, “hair damage”, “hair biology”. Skip advertising websites that promote a specific product.

Following are some information collected from different sources. Each text color represents a different source:

After it has been soaked for some time and absorbed the maximum amount of water possible, hair will be up to 20% weaker than dry hair. This means that wet hair must be handled much more carefully than dry hair. Source

Toxins affect hair strength. Source

Get Horsetail and other super green food sources in diet to increase hair strength. Source

This light daily Conditioning Rinse restores moisture and adds tensile strength to hair in just seconds. Source

Bleaching and dyeing change hair structure too, because the dyes and the bleaches used have to penetrate the cuticle and get into the cortex where they have their effect. Some degree of chemical damage is unavoidable.

All hair is dead. This isn’t a value judgment — it’s just a fact. Hair is composed of protein cells that die before they ever see the light of day. Any claim that a product can nourish or permanently repair hair is false, because you can’t revive a dead cell.

Is your hair looking worse for wear? Maybe you haven’t been caring for it properly. Watch out for these infamous hair harmers and – in the process – restore some vitality and life to your tired locks

Chemicals: Bleach, permanent and semi-permanent colors, perms and straighteners all work by swelling the hair cuticle to get underneath it. This roughens the scales of the hair cuticle and leaves them prone to chipping or coming off altogether. Bleaching, perming and straightening also break down some protein bonds in hair, weakening its structure.

Physical wear: Friction and daily wear and tear are, surprisingly, responsible for most hair damage. A rough brush or comb (or even a good brush or comb used too often), not rinsing salt or chlorine out of your hair after swimming or working out, and even the action of hair rubbing against a pillowcase can wreak havoc on hair. Other physical damage can result from abrasion of hairbands, hair clips and other accessories. When hair is wet, it is especially susceptible to breaking and splitting.

Heat: Repeated use of hair dryers, hot rollers, and curling and straightening implements can all crack cuticles and sap moisture from hair. However, adequate conditioning can insulate hair against low and medium heat, so make sure the settings on your hair implements are kept low.

Weather: Ultraviolet rays from the sun break down some of the natural protein bonds in hair. A little exposure to the sun’s rays won’t have too detrimental an effect on healthy hair (particularly if your hair is dark), but it can be pretty tough on hair that is chemically processed. Overexposure to wind can roughen up hair cuticles as well.

Each hair shaft is made up of two or three layers: the cuticle, the cortex, and sometimes the medulla. The cuticle is the outermost layer. Made of flattened cells that overlap like the tiles on a terra-cotta roof, the cuticle protects the inside of the hair shaft from damage.

To feel the cuticle, just pinch a single long hair between your fingers starting up near the root. Pull the hair between your fingers and feel how slick and smooth it is. As you move from root to tip, you’re running your fingers in the same direction as the cuticle layers. Now start at the tip of the hair. In this direction, the hair may feel rougher; it may squeak as it passes between your fingers. You’re running your fingers against the grain, and you’re bumping into the edges of all those flattened cuticle cells.

It’s handy to know how different conditions affect this protective layer on the outside of each hair. Chemists talk about solutions that are acidic (like vinegar or lemon juice) and ones that are alkaline (like a mixture of water and baking soda). In an acid solution, the cuticle cells shrink and harden. In an alkaline solution, the cuticle cells swell up and soften.

Underneath the cuticle is the cortex, which is made up of long proteins that twist like the curly cord on a telephone. Try stretching a hair and you’ll find that it’s elastic — it stretches before it breaks. When you stretch a hair, you are straightening the coiled proteins in the cortex. When you release the hair, the proteins coil up again. The pigments that give your hair its natural color are tucked among these protein strands and protected from the elements by the translucent layer of cuticle cells.

When you get split ends, you’re seeing the cortex at its worst. You’ve worn away the protective cuticle on the tips of your hairs with harsh treatment like hard brushing or too much sun and water. Without the cuticle, the fibers of the cortex fray like the strands of a rope. Since the cortex can’t heal itself, the only way to get rid of split ends is to cut them off.

In the center of some hairs is the medulla, a soft, spongy mass of tissue. Coarse hair generally has this layer, while fine hair usually doesn’t. The presence or absence of a medulla doesn’t have much to do with how your hair behaves when you wash or color or curl it, however, so you don’t have to worry about it.

To fill the gaps between the protective cuticle cells and to keep your hair shiny and flexible, glands adjacent to the hair follicle produce a kind of natural hair conditioner called sebum. Unfortunately, that sebum, which is an oil, also makes dirt stick to your hair. When you shampoo your hair, you wash away this protective oil and the dirt that clings to it.


Hair comes in many different forms: scalp hair, facial hair, body hair, and so on. In different parts of the body, hair varies both structurally and biologically. For example, scalp hair is different than the fine, downy, vellus hair found on the face of females or young people.

Hair also varies in shape, length, rate of growth, and response to stimuli. Factors such as light, hormones, temperature, and nutrition affect the rate of hair growth and the strength of the hair. The lengths of the phases in the hair life cycle also vary in different parts of the body. For example, scalp hair has the longest growth phase, and grows about 0.35 mm each day.

Hair Color

Melanocytes are pigment-producing cells. They are located throughout the body, and give skin including freckles and age spots and hair their color. Melanocytes produce pigments in the form of melanin granules. The melanoctye cells then transport the granules into other, surrounding kerainocyte cells.

The hair shaft gets its color from melanin of melanocytes at the base of the hair follicle. Keratinocytes then differentiate, die and are pushed upward to become the protruding hair shaft.

There are two kinds of melanin: eumelanin and pheomelanin. Eumelanin is present in hair that is brown, brownish-black, or black. Pheomelanin is present in lighter hair–blond and red hair. White or gray hair color occurs when the pigment-producing cells no longer produce melanin or the melanocytes are absent.

Coloring the hair is a rapidly growing practice. Estimates are that 50% of women color their hair and men are also changing their hair color. All age and ethnic groups are experimenting with hair colorants. Damaged hair and basic ethnic differences in the physical properties of hair have an effect on the penetration of the dye into the hair shaft.

Hair Structure

Hair is composed of three basic layers:
– the cuticle,
– a second, thicker layer called the cortex
– sometimes a third, inner layer called the medulla

The cuticle is the outer layer of protective scales. The cortex provides strength to the hair shaft, and determines the color and texture of hair. The medulla is present only in thick, large hairs.

The outer protective cuticle can be damaged by chemical or mechanical means, such as hair coloring or blow-drying. When the cuticle is damaged by such means, the protective scales are peeled away. This exposes the rest of the hair shaft. In some cases, even the innermost layer–the medulla–is exposed for further damage. Frayed scales of the cuticle impart a dull appearance to hair. An intact cuticle is responsible for the strength, shine, smoothness, softness and manageability of healthy hair. A layer of sebum coating the cuticle also adds to the shine. Conditioning agents can help to improve the look and manageability of damaged hair but cannot repair the shaft.

The root of the hair is inside the hair follicle. The root looks like an onion at the base of a plucked hair. In the root, the dermal papilla is the only true dermal part of the follicle. These cells play essential roles in regulating hair growth, hair cycle, and the size of the resultant hair. Surrounding the dermal papilla are epithelial keratinocytes and a smaller number of melanocytes.

Hair – Straight or Curly?

Different hair-fiber types result from differences in hair follicles. Thick, straight hair is produced by large, straight hair follicles. If you cut one of these hairs in half, you will see that it has a circular cross-section. Flat hair follicles make curly hair that has an elliptical cross-section. Curled follicles produce irregular kinky hair.

Hair shape is also an ethnic characteristic. Caucasian hair has an elliptical cross section giving it a slight curl while oriental hair has a circular cross section causing straight hair. Black hair has a flattened elliptical cross-sectional shape. This cross-sectional asymmetry accounts for the irregular kinky appearance of Black hair. Hispanic hair has a cross-sectional shape somewhere between a circle and a flattened ellipse giving it a wavy appearance.

Other factors affecting the shape of hair are the chemical bonds that form within hair proteins. Curl is a factor of disulfide bonds within the keratin molecules.

Elasticity of the hair varies but in general hair can be stretched 30% of its original length in water and experience no damage. Water is absorbed very rapidly causing the hair shaft to swell. The moisture content, elasticity and tensile strength is lower in Black hair than in Caucasians.

Reshaping or Styling Hair

Hair can be reshaped by styling, such as by permanent-wave styling or straightening solutions. These styling techniques perform three general steps:

1. The perm or straightening solution breaks disulfide bonds that determine the amount of curl in the natural hair.
2. The hair is mechanically reshaped into the desired configuration by using curlers, for example.
3. Neutralizing solution neutralizes the styling chemicals that broke the disulfide bonds, so that the disulfide bonds can be reformed in the new configuration.

This kind of treatment frequently weakens the tensile strength of the hair shaft.

There are other, less stringent methods of reshaping hair. The methods usually involve breaking the weaker hydrogen bonds that form between keratin molecules. For example, water disrupts some of these bonds. If hair is held in the desired shape while it dries (using curling rollers, sock rollers, etc), temporary changes can be made. High humidity can cause the hair to manifest its natural shape.

Hair Life Cycle

Hair originates in a follicle, which is a cavity in the skin that holds and protects the active or living cells that become the nonliving strand of hair. Each hair follicle operates independently of the other hair follicles. Because of this, each individual hair may be in a different phase of growth.

There are three phases in the hair life cycle:
– active growth phase, or anagen phase
– transition phase, or catagen phase
– resting phase, or telogen phase

Hair shedding usually occurs in the transition phase, but can also occur in the resting phase.

Active-Growth Phase

The hair root produces the cells that form the living part of the hair. This pushes the cells that already exist up and out from the follicle. As the distance from the follicle increases, the hair loses its nuclear DNA and becomes a strand of cross-linked proteins. This protein strand (the hair you comb or brush) is not living tissue. The only living parts of hair are the cells within the hair follicle in the skin.

Transition Phase

After a definite period of growth, the hair follicle goes into a transitional phase. New cells are not created at this stage. Instead, the hair follicle actually shrinks about 82%. Part of the hair root is destroyed, and the active dermal papilla breaks off from the rest of the hair follicle. This transition phase (from growth to resting) is called catagen, and lasts one or two weeks.

Resting Phase

In the resting phase, telogen, like the transition phase, the protein hair strand remains connected to the hair follicle, but it doesn’t grow. After five or six weeks, the dermal papilla reconnects to the base of the hair follicle and the bloodstream. The hair reenters the active-growth phase and a new hair begins to form.

The old hair strand is usually shed near the end of the resting phase. If it does not shed, the new active-growth phase pushes the old hair out (sheds it) to make room for the new strand of hair.

Normal Shedding of Hair

Hair is normally shed during the resting phase in the hair life cycle. When hair is shed, the protein hair strand is pushed out of the hair follicle to make room for a new hair. This is a part of a gradual replacement cycle. Normally, 50 to 100 scalp hairs are shed per day in the hair-growth cycle.

Different types of hair are shed after different periods of time. For example, eyebrow hairs lasts 3-5 months. Scalp hair lasts two to five years before being shed.

Baldness or alopecia results when replacement hair (new growth) fails to keep up with hair loss. Baldness or alopecia may be due to hereditary factors, pathological conditions, aging, or to radiation injury to the dermal papilli.

Hair Removal
Of course there are many methods of hair removal including shaving and waxing that are used on various body parts. Facial hair in women is usually dealt with by bleaching, waxing or electrolysis. However, a prescription cream that slows the growth of hair when applied topically is available to decrease unwanted facial hair. Eflornithine works by inhibiting an enzyme, ornithine decarboxylase, in human skin that ultimately affects the rate of hair growth. It does not remove hair and must be used continuously. It slows the hair growth so that less frequent hair removal is needed. Side effects may include redness, stinging, burning, tingling or a rash. It has not been evaluated for safety in pregnant or nursing women so it be avoided by these women.

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this investigation is to see how different factors affect the strength of the hair. We will test moisture, oil, chlorine, UV radiation, bleach and dye.

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

Independent variable for this investigation is the factor (moisture, oil, chlorine, UV radiation, bleach and dye).

Dependent variables is the tensile strength.

Controlled variables are the temperature, type and source of hair sample. We control these variables to make sure that our results is not affected by any of these variables.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

I think chlorine and bleach can reduce the tensile strength of the hair because these are both oxidizers and have a slow burning effect on hair. Moisture and oil however may act as a lubricant and reduce the mechanical damage. Lubricants may reduce the tensile strength, but will make the hair more flexible and resistant to mechanical stress.

Note that hypothesis must be testable. If for any reason you are not going to perform some of these tests, remove them from hypothesis.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”


For these experiments use hair samples about 5 to 10 inches long. It is best if the hair samples are from young individuals who have a healthy hair with no history of using dye and bleach. Use at least 10 identical hair strands for each experiment.

Expose the samples to each possibly effective factor and then measure the tensile strength.

How to measure tensile strength of hair samples?

To measure the hair tensile strength you may secure one end of a hair strand to something and then hang a weight to the other end of the hair strand. You will gradually add to the weight until the hair breaks from it’s weakest point. Total weight that caused the hair to break is called the tensile strength of the hair.

The clips that hold the ends of the hair may not in any way bend or damage the hair, otherwise hair may break from the contact point with clips and that will void the test.

Another method is pulling the hair using a pressure gauge or spring scale and record the final weight lead to breaking the hair. A spring scale up to 250 grams should be sufficient because human hair maximum tensile strength is about 150 grams.

You may use a spring scale to test the tensile strength of the hair. If you do so, you connect one strand of hair to the hook of the spring scale and then start pulling it down slowly. The largest weight that the spring scale will show is the tensile strength of the hair.

In physics when we measure the tensile strength, we measure it based on unit weight per unit area. In other words the diameter of the hair strand or wire that we test, affects the tensile strength. For example the tensile strength of a wire may be 300g/mm2 (300 grams per square millimeter). In this way we know that if we test another wire from the same material, but with cross-section area of 3 mm2 (3 square millimeter), then it’s strength will be 900 grams.

Since in this project we just want to compare the effect of some chemicals on strength, we can as well use the relative strength. So we do not measure the diameter of the hair and use total tensile strength of the sample as tensile strength of that sample.

Note that if you loop the hair around the spring, the force will be distributed in two sides and the measured tensile strength will not be accurate. However since you just want to compare tensile strength , you don’t have to be concerned about that.

You may also decide to divide the result by two in order to get to a number that is more close to the actual tensile strength of your sample.

Experiment 1: Testing the effect of bleach


Before you dye your hair, you use bleach to remove any previous die.

This experiment will test the effect of bleach on hair. Wear rubber or latex gloves during this experiment. Use 10 hair strands for this experiment.


  1. Keep five of the hair strands as control and do nothing with that.
  2. Wash 5 other hair strands with soap and warm water and then rinse it with warm water.
  3. Prepare the bleach solution as instructed by the manufacturer.
  4. Place five washed hair strands in the bleach and record the time.
  5. After 3 hours remove the samples from the bleach and rinse with warm water.
  6. Let all samples dry in the room temperature for at least one day. In this way all samples will be dry and have a moisture level balanced with their environment.
  7. Measure tensile strength of bleached samples and record the results. Calculate the average of tensile strength for bleached sample.
  8. Measure tensile strength of control samples and record the results. Calculate the average of tensile strength for control sample.
  9. Record the results in a table like this:
Treatment type Strength 1 Strength 2 Strength 3 Strength 4 Strength 5 Average
Nothing (Control)
Bleaching for 3 hours

Make a graph:

Make a bar graph to show the hair strength before and after bleaching. Use two vertical bars. Label them before bleach and after bleach. The height of each bar represents the average strength of hair samples on that category. Write the strength on the top of each bar.

Experiment 2:

Testing the effect of chlorine

Chlorine is used in tap water and pools as a disinfectant. You may have experience dull fuzzy hair after being in the pool for a while. This experiment will test to see if chlorine affects the strength of the hair.

Wear rubber or latex gloves during this experiment. Use 10 hair strands for this experiment.

  1. Keep five of the hair strands as control and do nothing with that.
  2. Wash 5 other hair strands with soap and warm water and then rinse it with warm water.
  3. Get a chlorine solution like the one used in laundry (Chlorox? or other ..).
  4. Place five washed hair strands in the chlorine and record the time.
  5. After 3 hours remove the samples from the chlorine and rinse with warm water.
  6. Let all samples dry in the room temperature for at least one day. In this way all samples will be dry and have a moisture level balanced with their environment.
  7. Measure tensile strength of chlorinated samples and record the results. Calculate the average of tensile strength for chlorinated sample.
  8. Measure tensile strength of control samples and record the results. Calculate the average of tensile strength for control sample.
  9. Record the results in a table.

Experiment 3:

Testing the effect of moisture

I am wondering if it is best to brush my hair when it is moist or when it is dry? This experiment will help me to get the answer. For this experiment use 15 hair strands.

  1. Keep five of the hair strands as control and do nothing with that.
  2. Wash 5 other five hair strands with soap and warm water and then rinse it with warm water. This will remove any oil and prepares the hair to be more moist.
  3. Use a hair dryer to dry the last five strands for 5 minutes.
  4. Place five washed hair strands in the chlorine and record the time.
  5. Measure tensile strength of moist samples and record the results. Calculate the average of tensile strength for moist sample.
  6. Measure tensile strength of dried samples and record the results. Calculate the average of tensile strength for dried sample.
  7. Measure tensile strength of control samples and record the results. Calculate the average of tensile strength for control sample.
  8. Record the results in a table.

Experiment 4:

Testing the effect of Ultra Violet radiation

A large amount of sunburn and skin cancer is believed to be from Ultra Violet radiation from the sun. UV lights with different wave lengths are available in Science Supply stores. A less specialized version of UV light known as Black light is available at hardware stores. In this experiment we test long term effect of UV on hair.

Use 10 hair strands for this experiment.

  1. Keep five of the hair strands as control and do nothing with that.
  2. Place 5 other hair strands under the intense radiation of a UV light. Cover the UV light and your test area with aluminum foil, so the UV radiation does not exit your test area. Continue the UV exposure experiment for 48 hours.
  3. Let all samples remain in the room for at least another one day. In this way all samples will be at the same temperature and moisture.
  4. Measure tensile strength of UV exposed samples and record the results. Calculate the average of tensile strength for UV exposed sample.
  5. Measure tensile strength of control samples and record the results. Calculate the average of tensile strength for control sample.
  6. Record the results in a table.

Materials and Equipment:

  1. About 200 strands of hair
  2. Bleach with manufacturer instructions.
  3. Laundry Chlorine solution
  4. warm water
  5. Latex gloves
  6. spring scale for tensile test
  7. Clips for holding the hair while doing tensile test
  8. Clock or timer
  9. UV light
  10. Wooden bar, paperclips, spring or other material that you may need to perform tensile test. (Do this based on what you can find)

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.

Record the results in a table like this:

Test Results for the effect of bleach on hair.

Tensile strength per hair strand Average
Bleached Sample 1
Bleached Sample 2
Bleached Sample 3
Bleached Sample 4
Bleached Sample 5
Control Sample 1
Control Sample 2
Control Sample 3
Control Sample 4
Control Sample 5


For tensile strength of each sample you need to calculate the average tensile strength of hair strands in that sample. Make sure to test at least 5 hair strands of each sample.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Following are some additional sample questions that can be studied.

  1. How does moisture affect the strength of the hair?
  2. How does UV radiation affect the strength of the hair?
  3. How do soaps and shampoos affect the strength of the hair?

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.


Visit your local library and find books related to hair. You may also find chapters or sections of biology books and dermatology books that discuss hair. Also find books that discuss material strength and methods of measuring tensile strength.

Review and use such books as your references in addition to this website and other online references.