This section is authored by John O’Hagan, Public Health England
Humans evolved with the sun as the main source of light. Therefore, daily activities took place when it was light: when the sun went down, we sought shelter and slept. Fire provided a form of artificial light, which was developed into lamps by burning oils and then candles. Despite this, one hundred years ago when houses had artificial light in the form of gas lamps, most people’s day was still driven by the availability of daylight. The incandescent light bulb and the installation of electrical supplies into factories and homes changed this, extending the day with sufficient levels of light to carry on complex tasks.
Since then lighting has changed. The focus on energy efficiency meant that the incandescent light bulb was phased out, moving to fluorescent lighting and then to LEDs. Fluorescent lamps provided one health concern. Linear fluorescent lamps were known to leak small amounts of ultraviolet radiation, managed using plastic diffusers to filter it or by the distance the lamps were from people. Following concerns from dermatologists, scientists at what is now Public Health England carried out an extensive study of the emissions from compact fluorescent lamps. This showed that some emitted levels of ultraviolet radiation could exceed the exposure limits for workers, especially when used close to the skin. A small
proportion of the population appeared to be particularly sensitive to these emissions. However, there were significant benefits to some people who needed a light source, for example, those needing a source close to the page of a book so that they could read.
Ideally, light should be controlled so that it only illuminates the areas where it is required – and only for the times when it is required. Light pollution is not new – the orange glow from sodium lighting above towns and cities has been a problem for decades. LEDs, coupled with well-designed optical systems, provide an opportunity to control light distribution, specifically to ensure that light goes onto the surface to be illuminated and not, for example, into the sky. There are also concerns that 24- hour light may have an adverse effect on flora and fauna.
Moving to a 24-hour society presents some challenges for our bodies. We evolved to experience a reddening sky as we move into the evening. Our melatonin levels should start to increase to prepare ourselves for sleep and to facilitate the body’s repair mechanisms. When we get up in the morning, the sunlight should suppress our melatonin levels, whilst serotonin production is increased to prepare us for activities of the day.
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In the early 2000s a type of sensor was discovered in the eye, in addition to the long known about rods and cones, which was also sensitive to light. Intrinsically
photosensitive retinal ganglion cells (iPRGCs) were identified as the main sensors for entraining our circadian rhythms. Humans have a natural body clock that has an approximate 24-hour cycle. However, light is the main trigger to ensure that we stay entrained. The initial research on iPRGCs, suggested that melatonin suppressed was most effective at a wavelength of about 480 nm (blue light). However, this wavelength is close to the peak wavelength known to cause adverse photochemical changes in the retina, which at high levels can result in eye injury. More recent studies have suggested that the rods and cones also contribute to the body’s response to light and circadian processes. Therefore, it is likely that bright light, of almost any wavelength, could have an impact. Disruption of the circadian system can have a major impact on sleep quality and daytime alertness, which in turn impacts wellbeing and safety. It is a bit like having permanent jet lag.
As artificial lighting technology developed, installers recognised the importance of ensuring the observer was shielded from high luminance (bright) sources of light because of glare, which in extreme cases can be very stressful. An obvious example of a shield is the lampshade used in the home. Some LED installations, however, have LED chips visible, which can form a source of glare. An extreme example is daylight-running lights on cars. These are clearly visible to other road users and pedestrians. At night, if they do not dim, they can be very dazzling and more so for young children (who have higher transmission of light through to the retina) and older people (who will suffer from scattering of the light, particularly in the lens of the eye). This means that older drivers, in particular, will be dazzled by oncoming vehicles with the risk that they may not see hazards until too late. The problem is exacerbated by fog.
Local authorities have been replacing mercury and sodium street lights with LEDs. If this is done purely on the basis of energy efficiency and cost, it is possible to end up with installations that may not be fit for purpose. Some streetlight luminaires have LED sources that can be seen physically projecting below the luminaire, becoming a glare source or light pollution. The light spectrum may be enriched in the blue, which may be beneficial for keeping drivers alert, but many people will find the light
uncomfortable. High levels of blue light are known to cause damage to the retina in the eye. This only tends to be a problem for blue LEDs and not for white-light LED sources containing a blue LED and a yellow phosphor. It is possible to have LED street lighting that directs the light only to the areas that need to be illuminated, minimising the light that goes in the sky. They can also be provided in a range of colour temperatures, where warmer colours are likely to be more appropriate for populated areas.
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Aside from the wavelength and brightness, there may be another impact of LED lighting. Some of the LED sources assessed by Public Health England and others vary in illuminance at a frequency of 100 hertz. At the extreme, the LEDs switch on and off 100 times per second. This is of concern for a number of reasons. Some people seem to be very sensitive to this light modulation, resulting in headaches, migraine and less specific feelings of malaise. However, most people will experience phantom arrays (as happens when you move your eyes quickly when behind a car with its brake lights on, particularly in the dark) and there is the risk of a stroboscopic effect. This effect may manifest itself as moving objects appearing to jump, rather than move smoothly. More seriously, rotating machinery, which could include the blades on a food mixer, may appear to be stationary if the rotation rate matches the modulation rate or is a multiple of it.
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