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UV Light—Slicing and Dicing the Light That Can’t Be Seen

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ultravioletlight2Several years ago, scientists hypothesized that a narrow spectrum of ultraviolet light called far-UVC could kill microbes without damaging healthy tissue. Far-UVC light at about 222 nanometers (nm) has a very limited range and cannot penetrate through the outer dead-cell layer of human skin or the tear layer in the eye, so it’s not a human health hazard. But because viruses and bacteria are much smaller than human cells, far-UVC light can reach their DNA and kill them. In the study, aerosolized H1N1 virus—a common strain of flu virus—was released into a test chamber and exposed to very low doses of 222nm far-UVC light. A control group of aerosolized virus was not exposed to the UVC light. The far-UVC light efficiently inactivated the flu viruses with about the same efficiency as conventional germicidal UV light.[1]

Hmmm … UVC? Perhaps it’s time for a refresher on the electromagnetic spectrum.

But first, why should you care? Weathering, of course. Many prepainted products spent their lifetimes outside in the sun, and many coil-coating formulators use accelerated weathering techniques to study weathering effects. When we use accelerated techniques, we are trying to learn—and learn more quickly—with the proviso that what we learn actually correlates with the real-life weathering of objects. More on that later; but regardless, it’s always good to have a solid understanding of the light that reaches Earth.

The electromagnetic spectrum covers a lot of territory. Our ability to see color starts with the visible light range, from blue light that has a wavelength of about 400 nanometers (nm) up to red light at 800nm. By the way, a “nanometer” is very small. A billion nanometers are in one meter. Below is a depiction of the electromagnetic spectrum, and note the wavelength scale to the far right is in meters.

photo 1

Beyond red is the longer-wavelength infrared. We don’t see it, but we sure feel it. It’s heat!


[1] https://www.sciencedaily.com/releases/2018/02/180209100701.htm?utm_source=feedburner&utm_medium=email&utm_campaign=Feed%3A+sciencedaily%2Ftop_news%2Ftop_science+%28ScienceDaily%3A+Top+Science+News%29

As wavelengths get longer and longer, they move from the infrared to the microwave range (handy for communications and making popcorn), and then to radio wave range (radar, television, and AM/FM radio). These latter waves are one meter long and longer. One meter is about three feet, so we’re talking about very long wavelengths. As an electromagnetic wave gets longer, it carries less energy along with it.

Going in the other direction (to the left of the blue range of visible light) is the UV region of the electromagnetic spectrum. The UV range is divided into three slices: A, B, and C, with wavelength ranges of:

  • UVA: 315nm – 400nm
  • UVB: 280nm – 315nm
  • UVC: 100nm – 280nmphoto2

As consumers, we are pretty familiar with the UVB region, which is mentioned in connection with sunburn, skin cancer, sunscreens, and SPF (sunscreen protection factor) values, which are a measure of a sunscreen’s ability to prevent UVB from damaging the skin. Although the UVB range goes down to 280nm, the Earth’s ozone layer, which sits from about 6 to 30 miles above the Earth’s surface, absorbs any UV radiation with a wavelength below 295nm. So the hazardous part of this UV range that we feel on Earth lies only between 295nm and 315nm, and this is the range where sunscreen absorbs UVB and protects our skin. Between this UVB range and blue light is UVA, but most sunscreens are not designed to protect against UVA radiation. Zinc oxide is an exception. The UVA range is not as energetic and damaging as UVB, but UVA rays penetrate the skin more deeply and are associated with wrinkling, leathering, sagging, and other light-induced effects of aging. Here’s a handy chart that describes various sunscreen ingredients and those wavelengths of light at which they absorb UV light.photo3

The UVC range lies between 100nm and 280nm. Ozone absorbs this entire range, and you won’t see any UVC until you reach outer space. It’s the UVC range, however, that caught my attention when I read the article about killing influenza virus. I was most surprised to learn that UVC cannot penetrate skin and eyes deeply enough to do any damage, but I don’t think the authors are suggesting routine exposure to UVC.

Okay, enough about the ABCs of UV light. Let’s look at some practical considerations for us in NCCA. ASTM has many standards on accelerated weathering tests. The four more-popular ASTM standards are:

  • G151 Standard Practice for Exposing Nonmetallic Materials in Accelerated Test Devices that Use Laboratory Light Sources
  • G154 Standard Practice for Operating Fluorescent Ultraviolet (UV) Lamp Apparatus for Exposure of Nonmetallic Materials
  • G155 Standard Practice for Operating Xenon Arc Light Apparatus for Exposure of Non-Metallic Materials
  • G90 Standard Practice for Performing Accelerated Outdoor Weathering of Materials Using Concentrated Natural Sunlight

If your company is a subscriber of the NCCA / ASTM Compass, you have easy access to these standards.

In the ASTM standards above (other than G151), the light sources used in the testing techniques are (for G154) a fluorescent light bulb; (G155) a xenon lamp; or (G90) mirrors that capture and concentrate natural sunlight. The latter technique is arguably the best at delivering real sunlight to test samples without any unrealistically short, destructive wavelengths of light. The xenon lamp—with standard, widely-accepted filters—matches the solar spectrum quite nicely. It’s the fluorescent bulb technique for which we urge caution.

There are two commonly-used fluorescent bulbs in G154:

  • UVA-340
  • UVB-313

The UVA-340 bulb has a peak emission at 340nm, and it does not emit UV light lower than 295nm. The UVB-313 bulb has a peak emission at 313nm. That might not sound so bad, since you already know that sunlight gets cut off below 295nm by the ozone in our atmosphere. The problem, however, is that the UVB-313 bulb, while having a peak emission at 313nm, does in fact emit UV light below 295nm at a significant enough level as to cause damage that might not normally be seen here on earth. See below.

photo4

https://www.sciencedirect.com/science/article/pii/S0141391008000657?via%3Dihub

Does this automatically mean that UVB is bad and UVA is acceptable? This is a question that cannot be answered simply. Work completed by NCCA 15 years ago indicated that no single accelerated test technique was suitable for all coil-coatings chemistries in all sorts of colors. However, even if you are only interested in one chemistry and one color, you still need to do the work to determine if you have acceptable correlation between the accelerated technique and real-time, in-service performance. In a future blog post, we will discuss an often-used rank correlation test.

David A. Cocuzzi, NCCA Technical Director

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