Residents of a condominium or housing estate in Singapore scurry to close all their windows when they hear “Brrrrrr ….” outside their flat or house. This is the familiar sound of the machine which spews out a thick cloud of insecticide to kill mosquitoes or bugs in the compound. When the dense white cloud has dispersed, the air is still laced with the lingering smell of chemicals for some time. I have always wondered what health hazard this fogging / fumigation poses.

A recent article titled “Residential and childhood leukemia: A systematic review and meta-analysis” written by researchers from the University of Ottawa caught my eye (Environmental Health Perspectives 2010; 118: 33 – 41). They reviewed and analysed previously published epidemiological studies examining the relationship between residential pesticide exposure during critical exposure time windows (preconception, pregnancy and childhood) and childhood leukaemia. The analysis found a positive association of exposure and childhood leukaemia. During pregnancy, exposure to unspecified residential pesticides, insecticides and herbicides carried 54%, 105% and 61% increased risk of childhood leukaemia, respectively. During childhood, exposure to unspecified residential pesticides and insecticides carried 38% and 61% increased risk of childhood leukaemia, respectively.

Most childhood leukaemia occurs in the first few years of life. Most leukaemics carry gross chromosomal abnormalities in their blood. A study in the late 90s on routinely collected blood samples in neonates showed the presence of leukaemic clones with specific chromosomal translocations in children who later developed acute lymphoblastic leukaemia (ALL) [Proc Natl Acad Sci USA 1997; 94: 13950 -13954]. Another study suggested that pre-leukaemic clones may persist throughout childhood and may require post-natal exposure for leukaemic progression (Genes Chromosomes Cancer 2004; 39: 335 – 340). A recent study of infants born in an agricultural region in the Philippines showed that the prevalence of a common acute myologenous leukaemia (AML) translocation [t(8;21)] in cord blood samples was about doubled in those infants with detectable meconium levels of the methylcarbamate insecticide propoxur (Pediatr Blood Cancer 2007; 49: 624 – 628).

Leukaemia is the most common form of childhood cancer in Canada and the United States. ALL accounts for about 80% of childhood leukaemic cases. The next most common is AML. Although the precise mechanism on how leukaemia develops is still unclear, the two-step leukaemia model proposes that leukaemia development occurs after 2 mutations. The first mutation, usually a chromosomal translocation, occurs in utero. The second mutation, occurring after birth, completes the process and leukaemia occurs. While ionizing radiation is the only proven environmental risk factor to play a role in the etiology of leukaemia, other potential risk factors include parental smoking and alcohol consumption, electromagnetic field exposure, hydrocarbons, socioeconomic factors, immunity and infection and pesticides.

While it has not been convincingly proven that pesticides / insecticides cause leukaemia, it is probably no harm that we avoid unnecessary exposure to these substances as much as possible. While fogging / fumigation helps to reduce the mosquito population, it may bring nastier problems in its place to you and your family.

Long-term exposure to low-dose ionising radiation has been linked to the development of solid cancers and leukaemia. Consequently, workers in the healthcare sector and in the nuclear industry who are at risk for repeated radiation exposure have to be monitored. They are limited to an effective radiation dose of 100 mSv every 5 years, with a maximum of 50 mSv allowed in any given year. With the increasing use of radiological scanning in the general population, patients are not monitored for their overall exposure to ionising radiation and there is little information on the subject.

Researchers from Emory University recently conducted a study on 655,613 enrollees, aged 18 – 64 years, from 5 healthcare markets: Arizona; Dallas; Orlando, Florida; South Florida and Wisconsin (N Engl J Med 2009; 361: 849 – 857). The amount of radiation exposure to each patient over the period January 1 2005 to December 31 2007 was noted. The mean cumulative effective dose of radiation exposure from imaging procedures was 2.4 +/- 6.0 mSv per person per year. Women underwent significantly more imaging procedures than men; 78.7 % of women undergoing at least one procedure during the 3 year period as compared to 57.9% of men. The proportion of enrollees undergoing at least one procedure during the 3 year period increased with age as well: 49.5% aged 18 – 34 years versus 85.9% aged 60 – 64 years. While most of the patients studied had low exposure (≤ 3 mSv per year, equivalent to the background level of radiation from natural sources in the United States), not an insignificant number of enrollees were exposed to higher radiation doses. Moderate annual effective doses (> 3 – 20 mSv) were incurred at an annual rate of 193.8 / 1000 enrollees, while 18.6 / 1000 enrollees and 1.9 / 1000 enrollees had high (> 20 – 50 mSv) and very high (> 50 mSv) exposure, respectively. The top 5 imaging procedures with the largest contributions to the cumulative effective radiation dose were myocardial perfusion scanning (22.1%), CT of the abdomen (18.3%), CT of the pelvis (12.2%), CT of the chest (7.5%) and diagnostic cardiac catheterization (4.6%). From this study, it was estimated that approximately 4 million non-elderly adult Americans would be receiving > 20 mSv per year from imaging procedures.

Part of the reason why Joe Public requests various imaging procedures during health checks is the perception that, if the various scans are normal, he is safe and free of cancer. He is only safe at that point in time. There is no guarantee that a small cancer is already lurking somewhere in the body. The scan could not demonstrate it because it was too small for the machine to ‘see’. Furthermore, there is no stopping a new cancer from developing 3 months after the scanning procedures. Imaging procedures should be ordered and used judiciously. While the doctor looking after the patient has a duty not to over-investigate, the patient must also refrain from pressurizing the doctor into doing a scan just because the patient saw a health report on TV about how powerful XYZ scan is in detecting cancer of the little toe!