Actions are already being taken to minimize the harmful effects of asbestos, lead, and many other recognized environmental health hazards, but what about hazards where the evidence for harm is not as clear? The possible impact of artificial light at night is an example of this particular challenge to policy and decision-makers.
Artificial light is everywhere. All night long, light from powerful street lamps and brightly lit office buildings reduces much of our exposure to darkness (Figure 1). Globally, artificial light at night is a very common phenomenon (Figure 2). Photographs taken during the night of a power blackout and on a typical night illustrate just how common artificial light is in our environment (Figure 3). In fact, when an earthquake knocked out the power in Los Angeles in 1994, “many anxious residents called local emergency centres to report seeing a strange ‘giant, silvery cloud’ in the dark sky. What they were really seeing—for the first time—was the Milky Way, long obliterated by the urban sky glow.”1
Referring to light as pollution may seem unusual, but there is mounting evidence that exposure to light at night is hazardous to our health. Several well-controlled studies have shown an association between breast cancer and shift work.23456 A variety of risk factors, including lifestyle and diet, contribute to differences in breast cancer rates worldwide, but researchers have proposed that breast cancer rates may be higher in industrialized countries because of the greater use of artificial light. One study in Israel overlaid maps based on cancer registries with satellite images of the earth and found that areas with greater light pollution also had higher rates of breast cancer.7 The researchers reported that the breast cancer rates in areas with average artificial night lighting were 37% higher than rates in less well-lit areas.
This type of ecological study, which looks at areas and groups rather than individuals, can be useful for identifying new patterns of illness, but the results should be interpreted with caution. A causal relationship cannot be inferred from an ecological study because an association between two variables observed at the group level does not necessarily mean an association exists at the individual level. An erroneous inference based on associations found in an ecological study is called an ecological fallacy. For example, it could be that light pollution is not a cause but a surrogate indicator for another aspect of life in an industrialized country that contributes to higher breast cancer rates, such as exposure to air pollution.8 Additionally, it could be that the individuals exposed to the most light pollution are not the ones with the highest rates of breast cancer, so making an inference from group-level observations to individuals can be misleading.
- 1. Chepesiuk R. Missing the dark: Health effects of light pollution. Environ Health Perspect. 2009;117:A20-A27. http://ehp03.niehs.nih.gov/article/info%3Adoi%2F10.1289%2Fehp.117-a20
- 2. Salhab M, Mokbel K. Breast cancer risk in flight attendants: an update. Int J Fertil Womens Med. 2006 Sep-Oct;51(5):205-7.
- 3. Linnersjö A, Hammar N, Dammström BG, et al. Cancer incidence in airline cabin crew: experience from Sweden. Occup Environ Med. 2003 Nov;60(11):810-4. http://oem.bmj.com/content/60/11/810.long
- 4. Rafnsson V, Tulinius H, Jónasson JG, et al. Risk of breast cancer in female flight attendants: a population-based study (Iceland). Cancer Causes Control. 2001 Feb; 12(2):95–101. http://www.springerlink.com/content/j0w883vt04717w72/
- 5. Reynolds P, Cone J, Layefsky M, et al. Cancer incidence in California flight attendants (United States). Cancer Causes Control. 2002 May;13(4):317-24. http://www.springerlink.com/content/mmy44phcgjgyfmg4/
- 6. Schernhammer ES, Kroenke CH, Laden F, et al. Night work and risk of breast cancer. Epidemiology. 2006;17:108-111. http://journals.lww.com/epidem/pages/articleviewer.aspx?year=2006&issue=...
- 7. Kloog I, Haim A, Stevens RG, et al. Light at night co-distributes with incident breast but not lung cancer in the female population of Israel. Chronobiol Int. 2008; 25:65-81.
- 8. Kloog I, Haim A, Stevens RG, et al. Light at night co-distributes with incident breast but not lung cancer in the female population of Israel. Chronobiol Int. 2008; 25:65-81.