In previous editions of BC Disease News, we have discussed, at length, the known effects of asbestos exposure on the development of mesothelioma, lung cancer, asbestosis, pleural thickening and pleural plaques.
Over the past couple of years, we have often made reference to asbestos in the same breath as ovarian cancer, in light of allegations, predominantly spouted in the US, that talcum powder-containing products were/are contaminated with trace levels of the human carcinogen. Readers can expect to be updated on this topic in weeks to come.
On one occasion (here), we also investigated whether asbestos could be associated with bile duct cancer and most recently (here), we explored a widely-publicised accusation that use of asbestos in pubs and breweries coincides with an increase in observed cases of oesophageal adenocarcinoma, the most common type of oesophageal cancer.
In this article, we report on the results of a new meta-analysis, published in the peer-reviewed Occupational & Environmental Medicine journal, in Q4 of last year, which assessed the impact of asbestos exposure on bowel cancer mortality.
The bowel is a component of the digestive system and is divided into the large bowel and the small bowel. The large bowel is made up of the colon, rectum and anus, hence why bowel cancer is interchangeably known as colorectal cancer, colon cancer and rectal cancer.
[Source: Wikimedia Commons – Blausen Medical Communications, Inc. (1997-2013): ‘Colorectal Cancer’]
According to Cancer Research UK, bowel cancer is the 4th most common type of cancer on a national scale (based on 2016 data), accounting for 12% of all new cancer cases (around 42,000 annually). Incidence rates have remained stable since the 1990’s, falling by just 2% over the past decade. Those most susceptible to the condition are typically aged 85 to 89.
Kwak et al (2019) evaluated 44 articles and 46 cohort studies (all published before April 2018) in the latest systemic review and meta-analysis on this topic.
The Korean researchers inferred that colorectal cancer mortality risk rises ‘significantly’ as levels of occupational asbestos exposure rises. While the effect size may have been ‘small’, mortality ratios for colorectal cancer were elevated in studies where the risk of asbestos-related lung cancer was also prominent.
In spite of the relationship observed, the study authors described the association between work-related asbestos exposure and colorectal cancer as ‘controversial’, given the conflict of opinion in previous studies.
Selikoff et al (1964) calculated a 3-fold excess risk of cancer of the stomach, colon, and rectum among 632 American insulation workers exposed to asbestos for at least 20-years. Similar conclusions were drawn in a larger study of insulation workers in 1979, but of a lesser magnitude.
Among British asbestos workers, Hodgson et al (1986) identified a significant deficit of colon cancer mortality, though shortly afterwards, Weiss (1990) deemed, in his review, that there was ‘a lack of coherence for the hypothesis that asbestos causes colorectal cancer’.
In the years that followed, both Gamble (1994) and Homa et al (1994) reported significant associations between asbestos and colorectal cancer, but failed to demonstrate the existence of a dose-response relationship, with the former having professed that:
‘The strongest evidence against a causal association between colon cancer and asbestos exposure is the lack of an exposure-response gradient in asbestos cohorts where trends for lung cancer are observed’. – ‘there is a consistent finding of an exposure-response trend for lung cancer (five of seven studies), and an equally consistent finding of no apparent exposure-response trend (six of seven studies) for CRC’.
In 2006, the US Institute of Medicine’s (IOM) Committee on Asbestos (2006), summarised that the evidence up to that point, had been ‘suggestive but not sufficient’ to infer a causal relationship between asbestos exposure and colorectal cancer:
‘The overall lack of consistency or of the suggestion of an association among the case-control studies (even those of the highest quality) and the absence of convincing dose-response relationships in either type of study design, however, weigh against causality’.
Re-evaluating the science in 2012, The World Health Organisation’s (WHO) International Agency for Research on Cancer (IARC) was ‘evenly divided as to whether the evidence was strong enough to warrant classification as “sufficient” for asbestos and colorectal cancer’, on account of ‘limited evidence’ that conveys an increase in risk.
Even a recent cohort study, conducted by Paris et al (2017), called for their findings to be interpreted ‘cautiously’, in view of an ‘insufficient number of [cancer] cases’. The group of French researchers examined around 14,500 asbestos-exposed men, but only made out an association between ‘cumulative exposure ... and the incidence of colon cancer ... with a time since first exposure of more than 40 years’.
Taking a broad view of the academic literature published over the past half-century, what is plainly noticeable is that, even when there the evidence has been indicative of an association between asbestos and colorectal cancer, it is consistently drawn back by the lack of correlation when exposure levels are low. This would suggest that there is no dose-response relationship, which would be uncharacteristic for asbestos (and carcinogens more generally).
 Kwak K et al., Exposure to asbestos and the risk of colorectal cancer mortality: a systematic review and meta-analysis. Occup Environ Med. 2019 Nov;76(11):861-871. <https://oem.bmj.com/content/76/11/861.long#> accessed 12 February 2020.
 ‘The Bowel’ (Macmillan Cancer Support) <https://www.macmillan.org.uk/cancer-information-and-support/bowel-cancer/the-bowel> accessed 20 February 2020.
 ‘Bowel cancer statistics’ (Cancer Research UK) <https://www.cancerresearchuk.org/health-professional/cancer-statistics/statistics-by-cancer-type/bowel-cancer> accessed 20 February 2020.
 Selikoff IJ et al., Asbestos exposure and neoplasia. J Am Med Assoc. 1964; 188:142-146. <https://jamanetwork.com/journals/jama/article-abstract/1162539> accessed 20 February 2020.
 Selikoff IJ et al., Mortality experience of insulation workers in the United States and Canada, 1943–1976. Ann N Y Acad Sci. 1979;330:91-116. <https://nyaspubs.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1749-6632.1979.tb18711.x> accessed 20 February 2020.
 Hodgson JT and Jones RD, Mortality of asbestos workers in England and Wales 1971–81.
Br J Ind Med 1986;43:158–64. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1007626/pdf/brjindmed00167-0014.pdf> accessed 20 February 2020.
 Weiss W, Asbestos and Colorectal Cancer. GASTROENTEROLOGY. 1990;99:876-884. <https://www.gastrojournal.org/article/0016-5085(90)90984-9/pdf> accessed 20 February 2020.
 Gamble JF, Asbestos and Colon Cancer: A Weight-of-the-Evidence Review. Environmental Health Perspectives. January 1995;102(12):1038-50 <https://www.researchgate.net/profile/John_Gamble6/publication/15486202_Asbestos_and_Colon_Cancer_A_Weight-of-the-Evidence_Review/links/5619040508ae78721f9cfc0f/Asbestos-and-Colon-Cancer-A-Weight-of-the-Evidence-Review.pdf> accessed 20 February 2020.
 Homa DM et al., A meta-analysis of colorectal cancer and asbestos exposure. Am J Epidemiol. 1994 Jun 15;139(12):1210-22. <https://academic.oup.com/aje/article-abstract/139/12/1210/141131?redirectedFrom=fulltext> accessed 20 February 2020.
 Institute of Medicine (US), Asbestos: Selected Cancers. (Chapter 11 Colorectal Cancer and Asbestos) (2006) <https://www.ncbi.nlm.nih.gov/books/NBK20332/pdf/Bookshelf_NBK20332.pdf> accessed 20 February 2020.
 IARC Working Group, ‘IARC Monographs on the Evaluation of Carcinogenic Risk to Humans: Arsenic, Metals, Fibres and Dusts (Volume 100 C)’ – see section 2.4.5 specifically (2012 IARC) <https://monographs.iarc.fr/wp-content/uploads/2018/06/mono100C.pdf> accessed 20 February 2020.
 Paris C et al., Occupational Asbestos Exposure and Incidence of Colon and Rectal Cancers in French Men: The Asbestos-Related Diseases Cohort (ARDCo-Nut). Environ Health Perspect. 2017 Mar; 125(3): 409–415. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5332175/pdf/EHP153.pdf> accessed 20 February 2020.