Interview with Kathryn Hatch, Ph.D, on How Consumers can Select and Care for Their Own Garments to Improve Their Sun Protection
Diana Clarke
Kathryn Hatch, Ph.D. is a professor at the University of Arizona College of Agriculture and Life Sciences, Agricultural and Biosystems Engineering Dept.
Tucson AZ.
Dr. Hatch has a special interest in what makes fabrics UV protective. In October 2001, the American Society for Testing and Materials presented her with an appreciation certificate for leadership in the development of standards for ultraviolet protective fabrics.
She has written numerous publications, including articles on UV protection including “Solar Protective Clothing. What’s Behind the Label?” AATCC Review, 5(8):31-34, 2005 and “Making a Claim that a Garment Fabric is UV-protective.“ AATCC Review 3(12):23-28, 2003.
Clarke: Can you explain how we would misperceive the light coming through fabric?
Hatch: Fabric is full of empty space (actually space filled with air). Some of these spaces are tunnels from the face to the back of the fabric. Such tunnels allow UV rays that enter the tunnel to pass though from face to back and strike the underlying surface (the skin). The UV rays that pass through are called direct radiation. The higher a fabric cover factor, the better. Cover factor and porosity are reciprocal concepts [1% porosity/99% cover factor].
So, let us say that a consumer is trying to decide whether there is a difference in the cover factor of two shirts (identical in style and fabric except one shirt is black and the other white). What comes to mind is to hold the two shirts up to the ‘light’ and determine how many pinpricks of light are coming thru the “tunnels” or estimate the percent of the fabric surface that contains no fiber – is empty.
The problem encountered is that consumer will indeed say the white shirt has higher cover factor than the black one, even though cover factor is the same) because the “light” has a higher contrast with the black than white surface.
Clarke: Is UV light and visible light transmitted through fabric differently?
Hatch: Yes, it is visible, not UV, light that is being assessed but remember the rays are coming though empty tunnels in the fabric, so it makes no difference what type of wavelength is involved for this assessment.
You can demonstrate this principle if you take a black square of paper and a white square, stack them, take a pin and punch a series of holes through both pieces. Separate the pieces and hold both up to a light. The “visual” effect is different.
Clarke: Most people don't want to wear long sleeved shirts and long pants in the
summer. Yet wearing shirts with sleeves to their elbows and pants to
their knees would at least provide better protection than shorts and short
sleeved or sleeveless tops. But what good would this be if UV light is transmitted through the fabric?
Hatch: Cover factor [surface area of fabric covered with fibers] is the MOST important factor in achieving Ultraviolet Protection Factor (UPF) ratings of at least 15 UPF, the lowest value for classifying a fabric as Ultraviolet protective. But iIf UV is passing through the fibers (not being absorbed by them), then UPF decreases even though cover factor is high. If the fabric does not have high cover factor and the garment does not cover the skin, it makes little difference what UV absorbing compounds make up the fabric!
Clarke: How sun protective are 50/50 cotton, polyester garments compared to 100%
cotton garments before they are laundered?
Hatch: A 50/50 cotton/polyester fabric will have a higher UPF rating than a 100% cotton fabric provided the fiber content is the only difference between the fabrics as polyester fiber absorbs UV rays better than cotton. Better can mean more of the identical wavelengths but also and importantly better at absorbing the wavelengths known to cause skin reddening. Fibers differ in their ability to absorb UV radiation – they differ in which UV wavelengths (remember there are multiple wavelengths of UV just like there are different wavelengths of visible light creating red, orange, yellow, blue, indigo, and violet light).
If one had identically constructed fabric, then the fabric made from polyester fibers would be more UV protective than the fabric made from cotton fiber.
Clarke: Why go through the trouble to find sun protective swim suits when so much
skin is exposed to the sun?
Hatch: The design of swimsuits has some influence as UV protective suits tend to cover more skin surface than “regular” swim suits. However, it is unprotected skin that will probably be the factor in becoming a skin cancer victim. Skin that is not covered is NOT protected. Garments styled to cover the most skin are better for sun protection.
Clarke: Are these true?
When fabrics shrink, UPF increases.
When fabrics stretch, UPF decreases.
Hatch: Shrinkage of fabrics during laundering tends to increase the UV protection provided by the fabric because it is the “tunnels through the fabric” that become smaller. So the likelihood of direct transmittance of UV light is decreased. Stretching a fabric may increase or decrease UPF value. Stretching a fabric, usually a knit, both crosswise and lengthwise, tends to decrease UPF (make a fabric less protective as the percent of surface area that is “ends of tunnels” increases. Stretching a fabric just crosswise (say a jersey knit) tends to increase UPF as the fabric tends to compact.
Clarke: Long term wear and fading could influence UPF value, too, yes?
Hatch: Exactly correct. Wear leads to thinning of the fabric (shorter distance thru the fabric for incident UV, and fading is due to loss of dye molecules (which absorb certain UV rays.
Clarke: Is it safe to say that if you are thinking of buying a particular garment
that comes in two colors, the darker one will absorb more UV, thus
providing more UV protection?
Hatch: There is no way to know what color of garment is more UV protective than others even when the fabrics are identical in all other ways. No, black fabrics are not always more UV protective than white ones!
Which UV absorbing compound [dye, for instance] and and how much of it (its concentration) on the fabric influences the “degree” of improvement in UFP value. Each Uv-absorbing compound has its own “finger print” in terms of exactly which UV rays are absorbed so they differ in effectiveness.
Clarke: So a consumer selecting Erika shorts of the same size, style, fabric composition, cover factor, and thickness but of different colors, won’t know which one has a higher UPF rating because the dyes may be different?
Hatch:
Absolutely correct!
Clarke: Would you say a thicker, heavier weight shirt does not always provide
more sun protection than a thinner, lighter weight shirt?
Hatch: All other factors being identical, the thicker the fabric the more UV protective it is.
Clarke: When tests are performed to determine a fabric’s UPF, the researchers seem to be trying to simulate when the sun is overhead, perpendicular to the swatch of fabric. Might the UV reflect off fibers at an angle at other times of day?
Hatch: Yes, determination of UPF is determined by directing UV rays perpendicular to a fabric’s surface. UV rays that happen to enter a “tunnel which is perpendicular to the fabrics’ surfaces pass directly through without encountering any fiber. In a wear situation, of course the UV is not always (perhaps not often) perpendicular to the fabric surface. The values reported on labels then do not over-estimate the protection.
Clarke: Does opaque fabric absorb UV?
Hatch: Opaqueness is not always an indicator of UV absorption.
Clarke: Rather than trying to select garments that are sun protective, would you say it is just much easier to buy Sunguard or wash garments several times in detergent with optical whiteners to increase the UPF?
Hatch: Consumers can add chemicals to cotton or other cellulose fiber containing fabrics (such as rayon and lyocell (Tencel) that enhance UV protection. Washing such fabrics in ‘regular’ laundry detergent results in the deposit of optical whitener chemical onto the fiber surface. The more washings a fabric undergoes, the higher the chemical build-up.
UV protection can be enhanced with the edition of chemicals to the fibers. As a consumer you do not usually know anything about the extent to which specific chemicals on the fabric enhance UV protection because they absorb the UV that strikes them. We know fiber content but not whether a fabric has been finished with an optical whitener, we do not know what dye(s) were applied etc. and do not know concentration.
If one of the chemicals from the UV-absorbing class is on the fabric ---- that fabric is highly likely to be labeled ---with a UPF value.
Even better is laundering cellulose-fiber fabrics with a laundry product containing a UV-absorbing compound [such as Sunguard]. One laundering in such products can give a high UPF value IF the fabric has a high enough cover factor. NOTE THE IF! The big difference between a UV absorbing compound and optical whitener is that the UV absorbing compound is much more effective in absorbing the most skin reddening UV rays.
These consumer added chemicals do not deposit on polyester, nylon etc.
People at high risk for skin cancer should really purchase and wear UV protective clothing, that class of clothing sold with a claim.
Clarke: How can a consumer determine from a label which detergents contain
Optical whitening compounds?
Hatch: Ingredients are listed on detergent boxes so yes, consumers can know which detergents contain optical whiteners. Almost without exception, optical whitening compounds are part of the detergent formulation.
Clarke: Will there be a sun protection rating for UVA like there is for UVB (SPF).
Hatch: Some apparel manufacturers who make a claim of UV protection will include information on the label that states the % UVB and % UVA rays that are absorbed by the fabric. Fabric that contains SunGuard is absorbing in the UVA as well as UVB --- the molecule was designed to be a broad spectrum UV absorber. Optical whiteners do not absorb effectively in the UVA.
Clarke: Are there any loosely woven fabrics that protect the skin better than
tightly woven fabrics?
Hatch: If you have a fabric composed of fibers (treated or not with optical whitener compound, UV-absorbing compound, etc) which absorb ALL UV rays striking them, the fabric still has to have high cover factor or the UPF of the fabric will be below 15 UPF and then would not be “classed” as a UV protective fabric. VERY HIGH COVER FACTOR IS ALWAYS REQUIRED.
Clarke: What do you wear in the summer? When did you start protecting your skin
from the sun and why? How do you select your clothing?
Hatch: I cover my skin or shade my skin with fabric as completely as possible – meaning weighing off UV protection and feeling “presentable.’ I use sunscreen for skin areas not covered with or shaded by fabric. I might wear a long sleeved shirt from car to office building and shed the shirt quickly once at destination. I have a couple of garments that were sold with a claim of UV protection – I rarely wear those because I do not spend long lengths of time in the summer sun. I like cotton knit fabrics. I launder them with SunGuard. I just try to minimize my exposure.
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Last modified: 06/23/07
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