Johann Ritter is best known for his discovery of ultraviolet light in A year earlier, in , William Herschel discovered infrared light. This was the first time that a form of light beyond visible light had been detected. After hearing about Herschel's discovery of an invisible form of light beyond the red portion of the spectrum, Ritter decided to conduct experiments to determine if invisible light existed beyond the violet end of the spectrum as well.
In , he was experimenting with silver chloride, a chemical which turned black when exposed to sunlight. He had heard that exposure to blue light caused a greater reaction in silver chloride than exposure to red light. Ritter decided to measure the rate at which silver chloride reacted when exposed to the different colors of light. To do this, he directed sunlight through a glass prism to create a spectrum. He then placed silver chloride in each color of the spectrum.
Nevertheless, there were isolated reports in the previous century of inhibitory effects of light. As mentioned above, Beclard , Schnetzler and Yung noticed that green light inhibited the growth and development of both vertebrate and invertebrate eggs, although the spectroscopic properties of the filters were not described.
Graber , Loeb , Hesse and Finsen 50 reported that various vertebrate and invertebrate animals avoided UV and violet-blue light if the intensity was too high. In , Marshall noticed that the motile larvae of sponges accumulated on the side of the tank with less light, and three years later Ultzmann found that isolated sperm survived for 48 hrs if protected from cold and light.
Early in the 20th century, the debate in the literature over the healthiness of heliotherapy and arc lamps provided the motivation for testing these ideas using animal models. The following studies are examples of pathological responses in animals that were induced by exposure to UV rays.
In most cases, the investigators employed high intensity artificial lights arc lamps, fluorescent lamps, lasers whose spectral emissions were enriched in UV rays. In those cases, the relevance of the results to sunlight is often unclear.
Circulatory and Immune System Damage. Similar results were obtained with visible light if the tissue was bathed in eosin or hematoporphyrin. The latter induced the formation of thrombii and localized leukocytosis, whereas UV rays alone induced only leukocytosis. Chronic low-dose solar-simulated UV radiation can cause both local and systemic immunosuppression , Suppression of the immune system may permit the outgrowth of UV-induced skin tumors.
Reproductive System Damage. In , Altenburg demonstrated that UV rays cause mutations in fruit flies if the rays reached the reproductive organs. One can only wonder whether other insects that are equally unprotected from sunlight and UV radiation are susceptible to similar damage and whether solar-induced mutations contribute to evolutionary changes. In , Findlay reported that skin tumors developed in depilated albino mice exposed for 8 months to UV rays from a quartz Hg vapor lamp.
Exposure of mice to the combination of UV rays and coal tar produced skin tumors in only 3 months. In , Roffo demonstrated that skin cancer could be induced in rats by exposure to either sunlight or Hg arc lamps. In , Funding et al. These results coincided with Latarjet's proposal that changes in atmospheric ozone levels could increase the risk of skin cancer.
In , Blum and associates , reported that skin cancer could be reproducibly induced in the ears of mice exposed to UV rays from arc lamps. A single exposure was insufficient, and cancer developed over time in a predictable fashion. Total irradiation dose was important, but not the exposure interval reciprocity held.
Only wavelengths below nm worked. Unlike humans, dermal tumors in mice were common. The authors speculated that this could be due to the greater UV penetration of mouse skin.
In , Freeman irradiated mice with a monochrometer at intervals between and nm and produced the first action spectrum for skin cancer. Using daily dosages equivalent to the threshold dose for erythema production in untanned human skin, he found that the peak carcinogenic response occurred at nm. His results supported the hypothesis that the carcinogenic effectiveness of UV rays is proportional to the erythema effectiveness.
He speculated that the two effects may have a common or similar site of action. In , Zigman and colleagues showed that longer wavelength UV rays from a "black light" are capable of inducing skin cancer in mice, a result confirmed by Strickland who also noted that UVA rays were far more carcinogenic when combined with UVB.
In , Setlow et al. The possibility that sunlight can cause mutations in skin cells leading to skin cancer has been supported by studies of tumor biopsies in humans and animals. Brash and colleagues , found mutations in the p53 gene in non-melanoma tumors in humans, and De Gruijl and associates reported similar mutations in mouse skin irradiated with UVB rays The incidence of tumors with p53 mutations is much higher in mice exposed to UVB rays, but approximately the same in mice exposed to UVA rays Since the p53 gene controls cell cycle regulation, a loss of function mutation in this gene could be an early event in the initiation of non-melanoma skin cancers.
Damage to Cultured Cells. Several investigators have noted that illumination of cells through a microscope caused deleterious effects. In , Uskoff noted that isolated white blood cells displayed greater outgrowth of processes during microscopic examination with red light compared with violet-blue light. They also noted that the mitochondria-specific dye Janus green was toxic even in the absence of light. In , Macklin reported that cultures of embryonic chick heart degenerate quickly when illuminated through a microscope using either daylight, tungsten globe, or a Welsbach burner.
As described in part 7 below , the result with the dyes probably involved the generation of toxic photoproducts due to the interaction of light with the dyes. Macklin and Kite showed that placing a filter between the light source and condensor reduces phototoxicity in cultured plant and animal cells.
The filter consisted of a glass vessel filled with a solution of dye copper sulphate or copper acetate that restricted transmission to wavelengths between nm actually ; see ref. In , Frederic showed that 90 foot-candles was damaging to cells when using violet-blue light and nm but not green, yellow or red light , and nm. In the presence of Janus green, he noted that even 4 foot-candles was toxic.
Curiously, these authors failed to cite the substantial literature on the toxic effects of light and dyes on other tissues and organisms.
It is unclear whether they were unaware of this literature, or whether they felt that it was so well known that it didn't need to be cited. Early prophase was the most sensitive period resulting in slower division. In , Wang et al. They provided direct evidence of single-strand DNA breaks and indirect evidence that production of hydrogen peroxide was involved. Damage to Excitable Cells.
The absorbance of UV radiation by nerve cells differed from the action spectrum of the response i. The absorption peak was between nm, whereas the peak of the action spectrum was around nm. This disparity led Booth and his associates to suggest that thiamin may be involved in the response. Chalazonitis showed that the photodynamic action of dyes on nerve cells resembled the effect of UV radiation alone suggesting a common mechanism.
They found dose-dependent effects on the conjunctiva, cornea, iris, and lens. At low doses, there was a slight conjunctival hyperemia but no effect on the other ocular tissues. At medium doses, haziness of the cornea developed. At high doses, there was edema and purulent exudation in the cornea and iris.
Upon microscopic examination, the lens capsular epithelium was swollen, and there was a ring of densely packed cells surrounding the exposed region. Some changes emerged hrs after irradiation including shedding of the corneal epithelial cells and leukocyte infiltration of the damaged areas. There was evidence of repair after days, and by 5 weeks all tissues exhibited marked recovery.
There was no noticeable damage to the retina even with very intense exposures. In , Ham et al. The violet-blue lines, but not the others, caused histological damage similar to that found in retinae from patients who gazed voluntarily at the sun for an hour before submitting to enucleation for malignant melanoma. Since light transmission through the lens peaks at nm, they argued that solar blindness is most likely caused by the shorter wavelengths of sunlight with possible thermal enhancement induced at longer wavelengths.
Over the next two decades, many investigators would lend support to their hypothesis that violet-blue light is the primary cause of solar retinopathy Indirect Effects Photosensitization. There are reports in the literature describing enhanced light sensitivity in ancient Egyptian and Indian cultures caused by injestion of certain fruits and vegetables.
There were, apparently, even attempts to treat various medical conditions using diet and light Nevertheless, the first scientific reports for such a relationship were noted by Dammann in and by Wedding in They reported that animals that ate buckwheat in the sunlight developed bubble-forming rashes on their skin only in areas lacking pigmentation.
Wedding hypothesized that sunlight caused a chemical reaction with the buckwheat as it traversed the cutaneous blood vessels in non-pigmented areas. This caused quite a stir and even the famous scientist Virchow expressed reservations about this interpretation Over time, additional experiments supported Wedding's idea, and eventually the scientific community embraced it. The first kind of supporting evidence came from an unlikely source.
Raab found that Paramecia stained with the fluorescent dye acridine red were killed when exposed to visible light. He also showed that animals treated with eosin and exposed to visible light suffered from edema and necrosis in the irradiated area.
While investigating the cause of the toxicity, he found that neither the light nor the dye was toxic when given alone. Furthermore, the dye was non-toxic if exposed to light separately and then applied. He concluded that it was the combination of dye and light that was responsible for the effect. Between , von Tappeiner Raub's mentor , Jodlbauer, and their colleagues went on to show that this toxic effect which they called "photodynamic sensitization" could be produced using any fluorescent dye and any wavelength UV or visible that excited the dye.
This led von Tappeiner to propose that it was the emitted light that was responsible for the toxicity. In , Blum 3 reviewed the results of papers related to this topic, and he concluded that it was not the light but rather some chemical toxin produced by the interaction of light with the dyes. This effect, he pointed out, was clearly distinct from the direct effect of UV rays on cells. Photodynamic actions required a dye or some other chemical to interact with the light, and the response was dependent upon the presence of oxygen.
The latter was demonstrated by Straub who hypothesized that the photodynamic effect was due to direct oxidation of cellular constituents. Blum 3 surmised that cellular damage was an indirect effect caused by photooxidation of the dye resulting in the generation of a toxic by-product, probably a peroxide.
He also ventured that the photosensitivity of range animals feeding on either buckwheat or St. John's wort was due to the same kind of photochemical reaction. In , Hausmann sensitized mice to visible rays by injecting them with hematoporphyrin, a natural blood-borne molecule that absorbs violet-blue light.
He noticed lympocytosis especially near the surface muscles and speculated that damage to the blood vessels was the primary cause of the sensitization. In , Adler showed that visible light stimulated skeletal muscle if the muscle was sensitized with eosin. In , Earle , found that illumination of cultured mammalian cells fibroblasts and white blood cells through a microscope was toxic if red blood cells were present. He presumed that the red blood cells produced a toxic by-product when exposed to light.
Based upon Raub's observations, von Tappeiner predicted that the interaction of light with chemicals could be a useful tool in medicine. Although they reported some success, it would take most of the 20th century to verify the utility of "photodynamic therapies" Microorganisms, Sunlight, and UV Radiation.
Their existence and role as mediators of infectious diseases were established during the 19th century. Improvements in microscopy allowed scientists to visualize their morphology and behavior as well as to investigate the conditions under which they propagated. It was during this period that scientists discovered the influence of light on these tiny creatures. Unlike the narratives for humans and non-human animals described above, the damaging effect of sunlight and UV rays on microorganisms was noticed early on.
Pathological Responses. In , Schmarda reported that microorganisms found in stagnant water displayed different responses to light. Some searched for it; others fled from it; some grew in it; others were damaged by it.
None lived exclusively in the dark. In , Lessona observed that marine pteropods and heteropods avoided sunlight and only approached the ocean surface at night. In , Engelmann , obtained results that supported Schmarda's observations. He showed that the amoeba Pelomyxa became immotile upon illumination, whereas the photosynthetic alga Euglena was attracted to light. They noticed that direct sunlight inhibited the growth of microorganisms in test tubes containing Pasteur solution.
Illumination for several hours resulted in test tubes free of bacteria for several months if the tube was subsequently sealed with a sterile cotton plug. Additional tests revealed that the bactericidal action was dependent upon the intensity, duration and wavelength of sunlight violet-blue being the most effective , as well as on the availability of oxygen.
Over the next 20 years, their results were confirmed and extended by numerous investigators who employed various types of bacteria, growth media, and light sources. In , Tyndall was the first to confirm Downes and Blunt's observations, but he suggested that it might be due to suppression of bacterial growth rather than a killing action.
In , Jamieson agreed that sunlight had a bactericidal effect, but that it was most likely due to temperature elevation of the medium rather than a direct effect on the bacteria.
In , Duclaux and Arloing demonstrated that sunlight had a direct killing effect on pure cultures of Tyrothrix scaber and Bacillus anthracis , respectively. Duclaux noted different sensitivities to light between strains. In , Roux confirmed that oxygen was required for the bactericidal effect of sunlight on B. In , Gaillard found that sunlight was damaging to many kinds of bacteria and spores but not to molds or yeast.
He agreed that the rate of destruction was dependent upon the intensity of sunlight, the composition of the medium, and the presence of oxygen. In , Janowski showed that direct sunlight killed B. In addition, the effectiveness of sunlight was dependent upon the initial concentration of bacteria and independent of any effect on the medium. Koch reported that sunlight killed the tubercle bacillus. This result was obtained only when the irradiation occurred in the presence of air oxygen.
Dandrieu showed that sunlight had a sterilizing effect on water, and he recommended using artificial light as a means of sterilizing drinking water. In , Klebs noted in his "General Pathology" textbook that bacteria and other microorganisms grew best when shielded from light, especially sunlight.
He recommended having bushes removed from pastures suspected of harboring anthrax since bushes shield the bacillus from sunlight. In , Geisler used a prism and heliostat to show that sunlight and electric lamps were lethal to B. Using quartz test tubes, he demonstrated that UV rays were the most lethal, although longer wavelengths were damaging at higher intensities.
Buchner developed a very sensitive assay for cell death that allowed him to detect the killing action of direct sunlight in as little as 10 min. He ruled-out any contribution of infrared rays by exposing the cultures through 0. This led him to speculate that sunlight has a natural sterilizing effect on rivers, streams and lakes.
Between , Ward performed a remarkable series of experiments demonstrating superb technical skill and ingenuity. Using improved versions of Buchner's assay and Geisler's apparatus, he showed that violet-blue and near UV UVA rays were the most damaging part of sunlight on bacteria. He also noted that pigmented fungi were resistant, consistent with the notion that pigments serve as protective filters. Finsen 50 showed that sunlight concentrated by a lens and passed through the ear of a white rabbit was capable of bactericidal action.
In , Westbrook , showed that the bactericidal effect of sunlight was greatest at the surface of cultures, whereas bacterial growth was facilitated deeper in the medium due to elevated temperature and decreased oxygen availability.
In , Richardson showed that sunlight had a sterilizing effect on human urine, and that irradiation of urine in the presence of oxygen resulted in the generation of hydrogen peroxide. This, they suggested, might explain the bactericidal action reported by Ward. In , Bedford showed that UV light stimulated hydrogen peroxide production in culture medium.
This led him to suggest that the destructive action of UV light on bacteria is caused by the interaction of light with photosensitizers in the medium resulting in hydrogen peroxide production leading to irreparable damage to the bacteria.
Between , Bie , used a carbon arc lamp and liquid filters to confirm that violet-blue and UV rays were lethal to bacteria. He also noted that oxygen was not required for the UV effect In , Strebel showed that UV rays from cadmium and aluminum arc lamps were more powerful than sunlight for killing bacteria. Bang , reported that B. In , Hertel performed the first rigorous quantitative assessment of the effects of light on microorganisms.
Using a thermopile and galvanometer, he demonstrated that UV rays from an arc lamp are several orders of magnitude more lethal than visible rays. He also observed some interesting cellular behaviors in response to UV rays including avoidance, strange locomotory behaviors circular, screwing, and rotatory motions , cell contractions, and death.
This was the first demonstration of the mutagenic effects of UV rays. They noticed that upon removal of the irradiation, cell division was often accelerated. Henri found that egg albumin absorbs rays in the UV region leading him to suggest that the bactericidal effect of sunlight is proportional to protoplasmic absorption. Burge , however, killed bacteria with UV rays, extracted their enzymes, and found that the proteolytic enzymes were unharmed.
They demonstrated that continuous and intermittent exposures were equally effective reciprocity. Wykoff , reported that the energy required to kill bacteria with X-rays was times less than that required with even the most potent UV rays i. He calculated that only one in four million absorbed UV photons is capable of causing cell death.
In , Gates measured an action spectrum for the bactericidal effect induced by a Hg arc lamp. The action spectrum corresponded to the absorption spectrum of nucleic acids with a peak response at nm. He proposed that the bactericidal effect was caused by UV-induced damage to nucleic acids. He also noticed that cell division was more sensitive to UV rays than to cell growth. In , Hollaender reported that E.
The response at longer wavelengths was also different in that it displayed a threshold, temperature coefficient Q10 of 2, and caused retarded growth and other sublethal effects. Jagger and colleagues confirmed Hollaender's observation that UVA rays inhibited bacterial growth as well as cell division in the absence of exogenous sensitizing agents.
Webb 15 reviewed the literature showing that UVA rays cause lethal and mutagenic effects in microorganisms even in the absence of exogenous photosensitizers. In , D'Aoust and colleagues showed that flavins are endogenous photosensitizers which underly the damaging effect of visible light in bacteria.
Hartman reported that irradiation of E. This eventually led to the notion of nucleotide excision repair In , Kelner found that the survival of bacteria exposed to UV rays is higher if they are illuminated with visible light immediately afterwards called "photoreactivation".
This led to the discovery of the enzyme photolyase, a flavin-based enzyme activated by violet-blue light that repairs pyrimidine dimers Studies of DNA repair mechanisms in bacteria have contributed to unraveling the basis of certain human disease including xeroderma pigmentosum and Cockayne syndrome , There is also emerging evidence that binding of transcription factors to the promoter regions of genes can inhibit repair and create hotspots for UV photoproducts , Physiological Responses.
The physiological response of microorganisms to light was first noticed by Schmarda mentioned above , but the first rigorous studies were performed by Engelmann. In , he found that Euglena was attracted to light i. In , he demonstrated that phototaxis of other protozoans toward Euglena was due to light-induced production of oxygen in the latter In , he showed that photosynthetic purple bacteria congregated in the near infrared region of the spectrum, i.
He inferred that this was a region of absorption by a pigment with properties similar to chlorophyll he called it "bacteriochlorophyll" that was important for the photosynthetic growth of the bacteria. In , Loeb proposed that phototaxis of Euglena is due to differential stimulation of their pigmented eyespots stigma , rather than direct activation of the flagellum. In , Mast reported experiments indicating that phototaxis involves both the eyespots and the flagellum.
In his model, flagellar motion causes the bacterium to rotate; rotation, in turn, causes alternating exposure of a photoreceptor adjacent to each eyespot which periodically shades the photoreceptors producing a succession of on-off responses. The latter allows alignment of the axis of the bacterium to the light. In , Buder determined that Euglena oriented toward a light source in the direction of the light rays, rather than to the light intensity gradient.
Brucker observed that the threshold for phototaxis in Euglena was raised by light adaptation. Links cited in proposed a model for bacterial phototaxis which hypothesized that light-induced elevation of intracellular ATP activates the flagellar motor.
In , Beijerinck reported that chromogenic bacteria are attracted to light. Pieper found that blue green algae were attracted to light greater than nm, but were negatively phototaxic to light below nm.
Between nm, he found that the reaction was positive in dim light and negative in bright light. In , Metzner showed that non-photosynthetic spirilla became phototactic when impregnated with the photosenstizing dye eosin.
In , Manten proposed that phototaxis in purple bacteria results from the sudden decrease in the rate of photosynthesis upon leaving the light. In , Schlegel showed that purple bacteria, which are normally attracted to light, are negatively phototaxic if the intensity is too high.
In , Clayton reported that phototaxis of purple bacteria occurs in the absence of oxygen and carbon dioxide.
In , Zalokar found increased photocarotenogenesis in Neurospora fungus exposed to violet-blue light. In , Bialcyzyk reported that motile cells of Physarum slime mold avoided violet-blue light.
Most studies of UVA and violet-blue light responses have implicated carotenoids and flavins as molecular photoreceptors. In , Galston proposed the alternative "flavin hypothesis" in which riboflavin acts as a photosensitizing agent in the photooxidation and stimulation of the growth hormone auxin indole acetic acid.
Forty years later, Galland reported that flavins are still regarded as the most common photoreceptors in blue light responses, although carotenoids and pterins have been implicated in some cases. One of the more controversial discoveries is the observation that cells produce, transmit and perceive ultraweak electromagnetic radiation also called ultraweak photon emission, low-level bioluminescence, and bio-electromagnetism.
The controversy was instigated in by Gurwitsch who reported that dividing Paramecia emit weak UV rays luminescence that are capable of stimulating cell division in other Paramecia.
Using sensitive detection techniques, Popp and others have measured spontaneous emission of low intensity electromagnetic radiation visible and UV from many types of plant and animal cells including mammalian cells.
The significance of these emissions, typically photons per sec, is still under investigation. The discovery of UV radiation and its effects on living organisms was a gradual process that involved contributions from chemists, physicists and biologists.
When it became clear that UV radiation is a component of sunlight, there was much interest in whether it might be responsible for some of the effects of sunlight on living organisms. The cumulative evidence to date indicates that UV radiation has both beneficial and harmful effects depending upon the type of organism, wavelength region UVA, UVB or UVC and irradiation dose intensity x duration.
The biological data so far are consistent with the following general statements. In the case of UVC and UVB, the cause is direct damage to nucleic acids and proteins that can lead to genetic mutation or cell death. The mechanism underlying UVA damage is less well-understood, but it probably involves the generation of reactive oxygem molecules that can damage many different components of cells including nucleic acids and proteins. Second, low doses of UVA radiation can induce physiological responses in organisms probably by activating specific genes.
The mechanism underlying gene activation is unclear, and it is uncertain whether low doses of UVC and UVB radiation can induce similar responses. Third, many of the physiological and pathological effects of UVA radiation can be obtained with violet-blue light. This is most likley due to a common photochemical transduction process involving flavinoids and carotenoids. Acknowledgements - I wish to thank Fred Urbach, Thomas Coohill and an anonymous reviewer for their critical reading of the manuscript as well as helpful comments and suggestions.
I also wish to thank Dennis Valenzeno for his encouragement, advice and suggestions regarding the scope and content of this review. Reprinted with permission from Photochemistry and Photobiology, 76 6 , pp. Laurens, H. Duggar, B. Duggar , pp. McGraw-Hill Co. Blum, H. Princeton Univ. Press, Princeton, NJ. Hollaender, A. McGraw-Hill, New York. Giese, A. In Photophysiology Vol. Edited by A. Giese , pp. Academic Press, New York. Jagger, J. Setlow, R. Setlow Effects of radiation on polynucleotides.
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Archives d'ophthalmolgie 9, Widmark, E. Hygiea 3, Widmark, J. Verhandlungen Biolog. Vereins 1, and Hammer, F. Ferdinand Enke, Stuttgart. What is its nature? Since antiquity, philosophers, mathematicians, astronomers, and physicians have sought a solution to this enigma.
For two and a half centuries, a lively controversy existed between partisans of the wave theory, for whom light is a vibration traveling as a wave, against those of the particle theory, for whom light is a stream of particles. Finally, quantum physics, born at the beginning of the 20th century, reconciled the two hypotheses: According to the nature of its interaction with matter, light may be manifested in the form of waves or in the form of particles, the famous photons.
A wave is characterized by a wavelength; A photon possesses energy. But on what scale? Over what range? It is the year In a room with closed shutters, he works with a small opening to isolate a single ray of sunlight. In the stream of light, he places a glass prism: Via refraction, the light breaks down into a rainbow of colors: Red, orange, yellow, green, blue, indigo, violet. In reality, indigo does not exist in the spectrum, but Newton added it for good measure at that time, the number 7 was thought to be endowed with magical and mysterious properties.
Seven colors. In , the English astronomer William Herschel placed thermometers in the solar spectrum to measure the temperatures of the different colors. Beyond red, where the eye sees nothing, the thermometer kept rising.
Herschel had just discovered the first invisible light, infrared. And at the other end? There, the surprise was just as great when, a year later in , the German chemist Johann Ritter exposed a photographic plaque covered with silver chloride to the solar spectrum.
He realized that it reacted considerably beyond violet. There is a second invisible light—ultraviolet. A gradient, a scale, was still lacking. His famous experiment with fringes of interference permitted him to measure the wavelengths in question, which are the size of microns, thousandths of millimeters.
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