Light is a form of electromagnetic energy that moves in measurable waves. The human eye is only capable of seeing a small segment of the light spectrum. This is called visible light. There are shorter and longer wavelengths that are not visible to our eyes. Examples of this kind of light would be X-rays, cosmic rays, radio waves and infrared waves, just to name a few.
Ultraviolet (UV) rays are not visible to our eyes, but when they hit certain substances, UV light causes those substances to “glow,” or release energy in wavelengths we can see with our eyes. UV light is also sometimes referred to as black light. When exposed to UV light, different substances will glow, or fluoresce, in colors specific to that substance.
While this article deals with the use of UV light for rodent inspection, it may be interesting to know that this technique also is used by food processors and others to identify food quality defects and contamination, as well as some disease-causing organisms.
Black light becomes a useful tool for rodent detection because rats and mice are incontinent. This means rats and mice urinate and defecate frequently, whenever and wherever the urge hits them. Mice will deposit anywhere from hundreds to thousands of micro droplets of urine per day.1 Rats will deposit even more.
Since amino acids in urine will “glow” when exposed to UV light rays, black lights may be a useful tool for tracking these incontinent rodents. Here’s an overview of how black lights can detect rodent presence:
- Rodent urine typically is deposited as a series of droplets in a line, with the larger droplets first, trailing off to smaller and smaller droplets.1
- Since Norway rats and mice drag their tails behind them, when they urinate, a smeared line may be noticed through the urine stain. However, since roof rats carry their tails up and off the ground, you will not see this drag pattern when roof rats are involved.
- In heavily contaminated areas, urine deposits may form large patches that are surrounded by scattered smaller droplets of various sizes. Large patches that show a radiating pattern away from the main deposit are not usually caused by rodents. Human urine or a spill of some kind is the likely source of such splash patterns.1
- Rodents like to move along walls when they travel, so they will leave grease stains that also may fluoresce.
- Rodents also may sit on the edge of boxes or on pallets and urinate, with urine dripping down the sides that may also fluoresce, especially on cardboard and paper. In this case, the urine stain will look like a line of paint dripped from box to box.
FIND THE CENTER. Since rats and mice like things to be quiet and they like to be hidden, it is possible you may not see urine evidence on the outside of a stack of palletized product, but there could be evidence within the pallet stack. Inspect inside product that is stacked with a center well. This is where rodents will sometimes hide and their evidence can sometimes be seen. A solid block of cases on a pallet is less likely to have hidden rodent evidence than a stacking pattern with a center well.
In addition, heavy urine deposits can be visible to the naked eye and will give off a characteristic musty odor. Urine deposits often are accompanied by hairs, droppings, gnawed product and nesting material. Therefore, we recommend if you’re working with a black light to use a flashlight as well when inspecting for rodent evidence.
KNOW THE GLOW. The key to identifying rodent urine is where you find it and the pattern. If you see something fluorescing evenly along the edge of a carton or along seams; it is not likely rodent urine. An evenly distributed pattern of fluorescing, like stippling, also is not likely rodent urine.
Fluorescence along a runway may indicate possible rodent presence more than glowing isolated in the center of a surface, with no evidence leading to the edge of that surface.
Here are some other materials that will glow under a black light:
- Urine from pets will glow an amber color because of the protein waste product that is excreted.1
- Rodent urine will glow a bluish-white in color when fresh and a yellowish-white color when old.
- Rodent hair will glow a bluish-white color.
- Be aware, many other substances like mold, glue, bleach, certain inks and dyes, oil, refrigerant, transmission and hydraulic fluids, and grease lubricants also will glow when exposed to black light.
While black lights are a help in identifying rodents, they do have limitations. Since urine deposits on walls and floors can get covered by dust and debris and become difficult to see and even more difficult to confirm as rodent evidence, the black light is not a good tool to make wide-area, structural inspections.
The black light is most effective when used to confirm suspected rodent presence and contamination from their urine deposits on commodities previously suspected by visual inspection.
Also, we do not recommend using the black light as the sole indicator and determining factor in documenting rodent evidence. If you find evidence glowing, we recommend you use a flashlight and visibly look for hairs, droppings, damage and nesting evidence to confirm rodent presence and/or contamination. One or more of these signs often is present if product and/or surfaces have rodent contamination.
THE DARKER, THE BETTER. The darker the ambient light in the area where you are screening product, the more effective the black light is. Darken areas you will be working in if possible, or place material that you want to inspect in areas with subdued lighting.
For example, working inside a dark truck trailer body is ideal if spot checking incoming goods. Turn the trailer spotlight on the dock off when using the black light. 1
PICK A MODEL. Black light tubes are manufactured to different specifications. One kind of black light tube attracts certain flying insects and is typically used inside insect light traps, while another type of tube is used for inspecting for contamination. These light tubes are designed and built for different uses.
Two types of black lights are available: battery-operated and direct plug-in models. Battery-operated units offer greater flexibility than direct plug-in models. Plug-in models offer brighter illumination that may be needed in well-lit facilities where the lights are used and portability is less important.
PRACTICE, PRACTICE, PRACTICE. The more you use the black light tool, the more proficient you will become. When you find rodent urine contamination, let others see what it looks like under the black light so they can have a reference point with which to compare the next time they use the light.
SAFETY FIRST. One question that comes up from time to time is: “Will the UV light coming from black light equipment injure our eyes if we look directly at it?” We believe the short answer is no because the wavelength coming from these sources will not hurt human eyes without unusual, direct and extended exposure.
For example, on the MSDS for the Osram Sylvania 350BL lamps used in the Gilbert ILTs (15-, 20- and 40-watt lamps) under Section V. Health Hazards, it says, “There are no known health hazards from exposure to lamps that are intact.”2 More information included on the Gilbert Web site confirms that these lamps are rated well below half the American Conference of Governmental Industrial Hygienists (ACGIH) established allowable exposure standards in the workplace.2
With that said, the Spectronics Corporation states (referring to darker-appearing 365 BLB lamps used for contamination detection) in its “Technical Bulletin #103” on Safety Precautions For Use Of High-Intensity Long Wave Ultraviolet (Black Light) Lamps: “Personnel using high-intensity, long-wave UV lamps should avoid looking directly at the sources….”3 The bulletin also states, “It is a good practice not to shine high-intensity UV-A sources onto exposed skin….”
Spectronics recommends using protective clothing and eye protection. However, on the MSDS for these 365 BLB lamps (listed as 365 nanometer Mercury Tubes) used by Spectronics under Section 3 Hazards Identification, it says “There are no known health hazards from exposure to lamps that are intact.”
The authors are technical director, Waltham Services, Waltham, Mass., and director of technical services, Dodson Pest Control, Lynchburg, Va.
1Corrigan, R.M. — 2001 “Rodent Control: A Practical Guide For Pest Management
Professionals,” 355 pages.
3Source: www.spectroline.com; access
“Technical Bulletin #103."