Association between pesticide exposure and colorectal cancer risk and incidence: A systematic review

Abstract

Background: Studies investigating the association between pesticide exposure and colorectal cancer (CRC) risk have been inconclusive.

Objectives: Investigate the association between pesticide exposure and CRC risk through a systematic literature review.

Methods: CRC has the fourth-highest rate of cancer-caused death in the US after lung cancer, breast cancer in women, and prostate cancer in men. Here we have conducted a systematic literature search on studies examining the association between any pesticide exposure and CRC risk using PubMed, MEDLINE via EBSCO host, and Embase according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses checklist.

Results: Following the review, 139 articles were included for qualitative evaluation. Study participants were farmers, pesticide applicators, pesticide manufacturers, spouses of pesticide applicators, farm residents, Korean veterans of the Vietnam War, rural communities, and those who consumed food with pesticide residues. The studies' results were split between those with significant positive (39 significant results) and inverse (41 significant results) associations when comparing pesticide exposure and CRC risk.

Discussion: From our literature review, we have identified a similar number of significant positive and inverse associations of pesticide exposure with CRC risk and therefore cannot conclude whether pesticide exposure has a positive or inverse association with CRC risk overall. However, certain pesticides such as terbufos, dicamba, trifluralin, S-ethyl dipropylthiocarbamate (EPTC), imazethapyr, chlorpyrifos, carbaryl, pendimethalin, and acetochlor are of great concern not only for their associated elevated risk of CRC, but also for the current legal usage in the United States (US). Aldicarb and dieldrin are of moderate concern for the positive associations with CRC risk, and also for the illegal usage or the detection on imported food products even though they have been banned in the US. Pesticides can linger in the soil, water, and air for weeks to years and, therefore, can lead to exposure to farmers, manufacturing workers, and those living in rural communities near these farms and factories. Approximately 60 million people in the US live in rural areas and all of the CRC mortality hotspots are within the rural communities. The CRC mortality rate is still increasing in the rural regions despite the overall decreasing of incidence and mortality of CRC elsewhere. Therefore, the results from this study on the relationship between pesticide exposure and CRC risk will help us to understand CRC health disparities.

Keywords: Colorectal cancer; Environmental exposure; Occupational exposure; Pesticides.

Figure 1.

Preferred Reporting for Systematic Review and Meta-Analyses Flow Diagram Modified from the PRISMA Group Source.

Figure 2.. Forest Plot of the Associations (effect size: RR, HR, OR, SIR, or PMR and 95% CI) Between Certain Pesticide Exposure and Colon Cancer Risk.

The pesticides aldicarb, dicamba, fonofos, EPTC, imazethapyr, terbufos, and trifluralin show positive associations while alachlor, lindane, 2,4-D, butylate, and DDT show inverse associations between pesticide exposure and colon cancer risk. Only significant associations from epidemiological studies with CI reported have been summarized in this figure. Associations from biological sample measurement studies can be found in Table 5.

Figure 3.. Forest Plot of the Associations (effect sizes: RR, OR, SMR, SRR, and SIR and 95% CI) Between Certain Pesticide Exposure and Rectal Cancer Risk.

This figure shows that pesticides such as toxaphene, pendimethalin, dieldrin and aldrin, chlorpyrifos, chlordane, carbaryl, and 2,4-D and MCPA have been reported to have positive associations while alachlor and lindane has been reported to have an inverse association with rectal cancer risk. Only significant associations from epidemiological studies with CI reported have been summarized in this figure. Associations from biological sample measurement studies can be found in Table 5. *These studies analyzed the association of concurrent dieldrin and aldrin and concurrent 2,4-D and MCPA exposure on rectal cancer risk.

Figure 4.. Forest Plot of the Associations (effect size: RR, OR, or SIR and 95% CI) Between Certain Pesticide Exposure and CRC Risk.

We have summarized that pesticides such as heptachlor, lindane, fonofos, and acetochlor have been shown to have significant positive associations, 2,4-D has been shown to have a significant inverse association, and alachlor and chlordane (one study not shown, biological sample measurement study) have been shown to have both significant positive and inverse associations with CRC risk. Only significant associations with CI reported from epidemiological studies have been summarized in this figure. Associations from biological sample measurement studies can be found in Table 5.

Figure 5.. Forest Plot of the Associations (effect size: RR, OR, HR, SMR, PMR, SIR, PCMR and 95% CI) Between Pesticide Exposure from Farming or Agricultural Work and Colon Cancer, Rectal Cancer, or CRC Risk.

We have summarized that pesticide exposure from farming or agricultural work have been shown to have either weak or no associations with colon cancer, rectal cancer or CRC risk.

Figure 6.. Forest Plot of the Associations (effect size: OR, RR, SIR, SMR and 95% CI) Between Occupational Pesticide Exposure and Colon Cancer, Rectal Cancer, or CRC Risk.

We have summarized that occupational pesticide exposure has been shown to have either weak or no associations with colon cancer, rectal cancer or CRC risk.

Figure 7.. Forest Plot of the Associations (effect size: OR, RR, SIR, and 95% CI) Between Environmental Pesticide Exposure and Colon Cancer, Rectal Cancer, or CRC Risk.

We have summarized that environmental pesticide exposure have been shown to have either weak or no associations with colon cancer, rectal cancer or CRC risk.