But don’t throw them all out just yet. The study these headlines were based on simply doesn’t prove a link between these products and breast cancer. It does, however, tell us something interesting about people’s beliefs about cancer and how it affects their memories.
What’s the study about?
In this study, led by Julia Brody at the Silent Spring Institute in the US, researchers phoned 787 women who had breast cancer, and 721 women who didn’t, to ask them about what kind of cleaning products they used in the past, and how often. Then they compared the two groups to see whether women with breast cancer were more or less likely to say they’d used these products than women without the disease, and whether the amount they used was different. This type of study is called a ‘case-control’ study .
The more people that are involved in a study like this, the more reliable its results, and the less likely they are to be explained by chance, or by errors. Sometimes, researchers have to use small numbers of people, for example if the disease they are studying is rare. But breast cancer is not rare – it’s one of the most common cancer in the US and the UK. For such a common disease, the number of people in this study was very small. But this isn’t the only reason why the study doesn’t provide evidence that there’s a link.
Memory and beliefs
One common problem with case-control studies is that they often ask people to remember their use of certain products, or their behaviour, many years ago. This is a problem for two reasons – firstly, it can be difficult to accurately remember information like this when it happened such a long time ago.
The second reason is that what people say can be unconsciously swayed by their beliefs. For example, if people have heard about a link between a product and their disease, perhaps in the media, or if they believed it could be a cause of their disease, they might focus more attention on remembering their exposure, or even overestimate how much they were exposed to in the past. It’s a well-known problem, called ‘recall bias’ .
But the really interesting thing about this study is that Brody specifically set out to measure how much recall bias affected the results. As well as asking about women’s use of cleaning products, the researchers also asked a range of questions about their beliefs about the causes of cancer. The women were asked to rate how much they believed that four factors (chemicals and pollution, family history, diet and reproductive history) could increase the risk of breast cancer.
When looking at the whole group, Brody found that those who used the most cleaning products had twice the risk of breast cancer as those who used the least. Then she looked separately at women who thought chemicals contributed “a lot” to breast cancer risk, and found that the risk was three times as high for the highest users compared to the lowest. But in people who said it contributed “a little” or “not at all”, there was no association.
This is a clear sign that recall bias is having an impact on the results. Cleaning products were only linked to breast cancer risk in women who thought that chemicals and pollution cause breast cancer. In those who didn’t hold to such beliefs, the link disappeared.
Does this study rule out a link?
Although this study on its own isn’t strong enough evidence for a link, it’s still possible that recall bias doesn’t explain the whole effect seen in the study, or that other factors are at play. And some chemicals in household products have been studied in the lab, where there has been a suggestion they could affect cancer risk. But this is a far cry from showing they could have this effect in real life.
As the researchers say, there need to be larger studies where people report their use of cleaning products upfront, and are then followed up over time, to get a more accurate picture of the effect of these products. Prospective studies like this aren’t as prone to recall bias, and would be much more reliable. But until those studies are done, we won’t know whether or not there’s any link. For the moment, there’s certainly little good reason for concern.
Ami R. Zota, Ann Aschengrau, Ruthann A. Rude, & Julia Green Brody (2010). Self-reported chemicals exposure, beliefs about disease causation, and risk of breast cancer in the Cape Cod Breast Cancer and Environment Study: a case-control study Environmental Health, 9 (40) : 10.1186/1476-069X-9-40