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Preemies get more retinal irradiance
than safety guidelines allow for adults
ROP and the introduction of fluorescent light
Fluorescent lamps were first introduced commercially at the World Fairs of 1938/39 in San Francisco and 1939/40 in New York (26, 27) and proliferated in factories, hospitals, and other large buildings across the U.S. After the war, fluorescent lamps were welcomed in most other industrialized countries with equal enthusiasm. Once the Second World War was over and normal trade as well as factory building resumed, this new and money-saving, technology spread in all directions. Everywhere, hospitals were among the first to install the new lamps, since their bright light was considered helpful against dirt and germs.
The first 2 babies reported to have suffered ROP were born In July and November 1940 in Boston (28). The mysterious condition soon appeared in most intensive care nurseries across the U.S., and after 1948 in intensive care nurseries in many other industrialized countries.
In 1952, Dr. Leona Zacharias from the Harvard Medical School published a 219-reference literature survey of just about all that was then known about ROP. She documented the time of first occurrence in other countries: Israel -- 1947; Australia, Canada, and Sweden -- 1948; Switzerland 1949, Cuba, France, Holland. Italy South Africa, and Spain -- 1950 (29).
The U.K. was the earliest. Both fluorescent lamps and ROP made their debut there right after the war, according, to four reports presented at the 1951 session of the Ophthalmological Society of the U.K. Two of these reports described the First cases of ROP in 2 babies, one born in 1946 in Birmingham and the other in 1948 in Oxford. Both reports noted that the incidence of the disease increased rapidly (30, 31). The other two papers discussed experiments with fluorescent lighting in hospitals; one of the authors expressed his appreciation to the General Electric Company for their help in making the special fittings required for these lamps "so soon after the war" (32, 33).
Because the disease had appeared so suddenly, some physicians wondered if it had been there all along but had simply not been recognized before. They organized several large-scale retrospective studies on ROP among older blind people. Some of these studies found a few isolated and uncertain cases beginning, in 1937 (34, 35), but they all concluded that if ROP had existed before 1940 in the U.S.A., or before 1946 in the U.K., it must have been exceedingly rare (36).
When Dr. Theodore L. Terry first described the new disease in 1942, he postulated that "some new factor has arisen in extreme prematurity to produce such a condition" (28). In 1943, he argued that this new factor was excess light:
He expressed the same argument again several years later:
A year later, Dr. Terry's comments were read to the American Academy of Pediatrics:
Retinal vulnerability to fluorescent light
Fluorescent tubes contain a thin mixture of mercury vapor and some noble gases. Electromagnetic fields in the lamp accelerate ions to high speeds and energy levels. When these fast ions hit the mercury atoms, these emit high-energy streams of photons, mostly in two wavelengths in the ultraviolet region. To transform these into the longer wavelengths of visible light, the inside of the fluorescent lamp tube is coated with a layer of phosphor (Greek for "light bringer").
Phosphor absorbs light and then reemits that radiation spontaneously for hours and in a different color, as in a luminous watch dial. The photon bombardment from the excited mercury atoms inside the tube greatly multiplies and accelerates this reradiating glow. The photons emitted from the mercury enter the phosphor atoms and exit at a longer wavelength, using the phosphor atoms like so many launch-pads up into visibility.
Fluorescent lamps emit their light waves independently of each other, unlike lasers which emit them in-phase as coherent light. The dangers from laser light have received much more regulatory concern that those from fluorescent light, although both types of light are equally damage to the retina.
The light receptors in the retina absorb the energy from these waves one photon at a time, whether that photon arrives in step with others or as part of an unorganized group (40). Indeed, retinal damage from coherent and noncoherent light sources is similar. The experimentally derived threshold values are in fairly close agreement whether the light comes from non-coherent xenon lamps and carbon arcs or from coherent helium-neon, ruby, or argon lasers (41).
The lowest threshold value for light damage to animal retinae is reported for non-coherent blue light (42) like that from the most intense of the energy spikes in the fluorescent lamp spectrum.
When the photons emerge from the phosphor atoms in the fluorescent lamp, they shoot out in specific wavelengths and form intense spikes of concentrated energy radiation. These spikes occur in all fluorescent lamps at the same wavelengths 365.0 nm; 404.7 nm; 435.8 nm; 546.1 nm; and 578 nm -- and approximately with the same relative intensities (43). The differences between the different types of fluorescent lamps are mostly in the broadband spectrum reradiated by the different phosphor formulations.
The fluorescent lamps in intensive care nurseries are the "Deluxe Cool White" type, as specified by the Committee on Fetus and Newborn of the American Academy of Pediatrics in its 1977 Standards and Recommendations for Hospital Care of Newborn Infants (44).
The distribution of the energy from this type of lamp over the different wavelengths of the spectrum is shown in Figure 1 and is copied from Sylvania, a maker of these lamps. The corresponding curves for "Deluxe Cool White" lamps from other manufacturers look similar and feature the same narrow-line photon emission spikes.
Figure 1 does not show the full height of these spikes, since it averages the energies over bandwidths of 10 nm. The spike at 435.8 nm, for instance, is only 0.1 nm wide (45) and would appear almost 100 times higher on the graph if it was not averaged with the neighboring wavelengths. This spike packs 8.5% of a typical nursery lamp's total energy output (see Table 1).
Due to the higher photochemical energy of shorter wavelengths, this spike in the short-wave end of the visible spectrum accounts for an even higher percentage of the total photochemical activity produced by the lamp: in vitro experiments of bilirubin conversion by fluorescent lamps have shown that the single energy spike at 435.8 nm is responsible for more than 50% of the conversion reaction (46).
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