LRC research discounts satellite images as accurate light-level indicators

Sept. 16, 2011
Previous studies have used satellite images to link outdoor light levels and cancer rates, but Rensselaer Polytechnic Institute's LRC has published research that discounts satellite images as accurate indicators of terrestrial light levels.

The Lighting Research Center (LRC) at Rensselaer Polytechnic Institute (RPI) has published new research that counters previous studies that had indicated that satellite images of outdoor lighting were accurate predictors of breast cancer incidence. While the LRC didn't discount that the disruption of circadian rhythms could cause health problems, the researchers said details on the "spectrum, quantity, and duration of light exposure reaching the eye" would be needed to judge health impact and that satellite imagery can't provide those needed details.

“After shift work was identified as a probable carcinogen by the World Health Organization, some studies were published that claimed a statistical association between light at night and the incidence of breast cancer. However, these studies relied on satellite photometry and subjects’ self-reports of bedroom brightness as measures of light exposure. None of these studies employed actual light measurements at the eye,” said LRC Director and Principal Investigator Mark Rea. “Before statistical associations between light at night and disease can graduate to a cause and effect relationship, it is necessary to measure the light as a potential causal agent. Our study showed no relationship between the measured light on the ground and the measured light in space.”

Teacher wearing a Daysimeter

The LRC utilized light meters to take measurements inside and outside the bedrooms of 72 female school teachers. Those measurements were compared with satellite measurements of brightness at ground level. The tests were conducted in different geographic areas that ranged from low to high in terms of relative brightness of the sky.

The researchers also recorded light exposure over 24-hour periods for the test subjects for seven consecutive days. The subjects wore a Daysimeter, a light-measuring device developed by the LRC, on their head to record light-exposure levels day and night.

The researchers concluded that there is no link between the light levels people experience on the ground and the satellite measurements. Moreover they found that the test subjects experienced a normal 24-hour circadian cycle regardless of the sky brightness in their local area.

The RC was careful to state that its research doesn't absolve night light of health risks. Rea said, “It is important to note, however, that these findings do not undermine the foundational data using animal models that link melatonin suppression by light at night and cancer risks, nor does it contradict the statistical association between shift work and breast cancer risk in humans. Rather, these findings simply undermine the inference for a causal relationship between light at night, as measured by satellite photometry, and breast cancer incidence.”

On the subject of shift work, the researchers stated that they specifically avoided using shift workers as test subjects. They made that decision because shift work has been shown to create a higher risk of breast cancer.

The Centers for Disease Control and Prevention, the Trans-NIH Genes, Environment and Health Initiative and the National Electrical Manufacturers Association contributed funding for the research.