With Halloween on the horizon, there’s no more appropriate time to have a look at the International Agency for Research on Cancer’s (IARC) curious cancer determination for those who work the “graveyard shift.” But like any good Halloween horror film, it might seem a lot scarier than it actually is.
An estimated 15 million Americans, or 15 percent of the workforce, work at night. Many of the most common night shift occupations — such as nurses, emergency medical technicians, firefighters, police officers and air traffic controllers – are essential, often life-saving occupations. But could exposure to light after dark as part of these occupations cause cancer? According to the IARC, it’s probable.
Based on extremely limited evidence, IARC’s Monograph found “exposure to light at night disturbs the circadian system with alterations of sleep-activity patterns, suppression of melatonin production, and deregulation of circadian genes involved in cancer-related pathways.”
Limited, Porous, and Incomplete Evidence
In addition to citing highly limited evidence, some of the data IARC relied on were from studies that employed flawed methodologies and were not comprehensive. For example, some human evidence studies, cited by IARC, did not examine whether exposure to light at night causes cancer. Instead, these studies looked for an association between those who work at night and instances of cancer.
Furthermore, several of the studies relied upon by IARC failed to control for significant factors that could contribute to cancer, such body weight and alcohol consumption, which can ultimately influence study outcomes. Cancer Research UK notes these varying factors and limitations, and the difficulties in drawing “solid conclusions”:
“Although shift work did seem to increase breast cancer risk, some studies didn’t account for other risk factors, like body weight and drinking alcohol. Others didn’t use enough people to draw solid conclusions. And the different studies often used different definitions for ‘night shift’, meaning it’s difficult to compare the results.”
These limitations were also noted by numerous breast cancer experts, who urged the public and policymakers not to overreact to IARC’s determination.
“My personal view is that it is a complex Rubik’s Cube and we might be looking at this as a night-shift issue, but when you turn the cube the other way, it is probably obesity, it might be [hormone replacement therapy], it might be a lack of vitamin D, which would be my strong guess.”
Kat Arney, Ph.D, said of IARC’s classification of shift work,
“At the moment, we don’t know how other lifestyle factors — such as taking [hormone replacement therapy], obesity, having fewer children, and alcohol consumption — interact with shift work to increase a woman’s risk of breast cancer.”
Evidence Contradicting IARC Keeps Mounting
As researchers continue to study the impact of light at night, evidence mounts that contradicts IARC’s determination. Recently, in a 2016 study, researchers at Oxford University found no evidence that nightshift work leads to increased chances of developing breast cancer.
The study, called “the largest of its kind,” pooled the seven studies IARC used to make its determination with an additional three UK-based studies of 800,000 women, bringing the grand total of study participants to 1.4 million.
As lead researcher Dr. Ruth Travis of the Cancer Epidemiology Unit at Oxford University said:
“We found that women who had worked night shifts, including long-term night shifts, were not more likely to develop breast cancer, either in the 3 new UK studies or when we combined results from all 10 studies that had published relevant data.” (emphasis added)
After the University of Oxford study was published, Dr. Lindsay Mcowat, who works many night shifts said, “This new research leaves me feeling relieved that I don’t have to worry about breast cancer too.”
While there are plenty of reasons to have a spooky Halloween this year, there doesn’t seem to be a reason to be alarmed by IARC’s determination on the “graveyard” shift.