Looking at the newspapers this morning, you might end up wanting to rush home and clear out all your cupboards of the products you use to clean your home.
The headlines have ranged from “Household cleaners may double risk of breast cancer”, all the way through to “Could being too houseproud raise the risk of breast cancer?”
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 said they used the most cleaning products had twice the risk of breast cancer as those who remembered using 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.
- This story has also been covered in depth on the NHS Choices Behind the Headlines blog
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
Katia August 3, 2010
Can someone please clarify how the chemicals from cleaning products can cause breast cancer? To do that they would need to get into one’s body. If one wears gloves while cleaning then there is no contact with the chemicals…Unless small residue left on the surface is ingested..? Or perhaps inhaling the smells of products may cause the damage?
steve July 28, 2010
Surely is there was a link it would show up first and most clearly in employees of the manufacturers and in people whose occupation involves long term use of such cleaning materials.
The Reactionary Researcher July 27, 2010
Mary, The issue regarding the funding of cancer research has less to do with what cancer researchers are willing to do in terms of research, and more to do with what funds are actually made available to the researchers. It’s always a question of what funding is available. I can guarantee you this: if you make funds available for cancer researchers to investigate the issue of cleaning products, they’ll line up to do the studies and spend the funds.
If these cleaners do cause cancer, and there’s no funding to do the studies, I guess we’ll never know.
Quite frankly, there’s not much to take seriously in this study. A phone survey of Cape Cod residents, asking them to recall their use of more than 25 different product types over the long term, that correlates to the incidence of cancer isn’t science. It does nothing to suggest whether or not these products are actually an issue; there are simply too many variables to take this study seriously. Sorry.
The Reactionary Researcher
Mary July 25, 2010
I think the real issue is whether cancer research are going to spend funds to research whether there is a link between cancer and household cleaning products. Are you going to investigate. You talk about being there for people, to support the prevention of cancer. What if these cleaners do cause cancer, everyone uses these, apart from environmentally consious people who use organic products. I think you are trying to dig holes in this study instead of taking it seriously and taking action to potentially protect the mass population. Household cleaners are absolutely packed full of harmful chemicals, they are toxic. Of course they aren’t good for the body. Waken up and do something about it!!!!
The Reactionary Researcher July 22, 2010
Good analysis. It’s nice to see another person actually reading and evaluating the study on its merits. I was a bit more harsh in my own analysis, but I appreciate and respect your opinion.
Jess Harris July 21, 2010
Mills – thanks for your comment. You’re right that recall bias is something we should always consider when looking at a case-control study, and we discussed the issue in the sunbeds post you refer to (in the section called ‘remembering past exposures’). But there are a couple of things that make these two studies different in terms of how strong their conclusions are:
– The sunbeds one adds to a large pile of evidence which is pointing to the same thing. IARC have already ruled sunbeds are ‘carcinogenic to humans’, and there have been many studies, both case-control and prospective, which the authors of the cleaning products study say would be needed to provide evidence. So on the whole, the balance of the evidence shows that sunbeds increase melanoma risk, whereas there isn’t any evidence in humans saying cleaning products affect breast cancer risk at this point.
– In the cleaning products study, the authors specifically did another analysis to see whether and to what extent recall bias could be affecting their results, and found that it had a strong impact. However, other studies haven’t found much evidence of recall bias in sunbed reporting. So there’s evidence that the extent of the problem differs between these two studies.
It’s important to consider studies in the context of what’s already known about the area, and to consider the strengths and limitations of the research. In the case of this story, the authors’ own words about it are “The modest association and possibility of recall bias make interpretation tentative.”
Mills July 20, 2010
Case-control studies and the impact of recall bias are a problem. That is why it’s surprising that the recent blog “Sunbeds and skin cancer – the evidence stacks up” was so gushing over results from a case-control study.
“Confirmation bias” (en.wikipedia.org/wiki/Confirmation_bias) perhaps?