Childhood leukaemia and night-time exposure

The Ahlbom pooled analysis found a raised relative risk for childhood leukaemia for 24-hour exposure in the home greater than 0.4 µT:

Field

<0.1 µT

0.1-0.2 µT

0.2-0.4 µT

>0.4 µT

Relative risk

1

1.08

1.11

2.00

 

A year later, a new study from Germany found a similar result for 24-hour exposure, but also found the same thing as a smaller, earlier German study, that there was a higher relative risk for exposure just during the night-time.  The results from these German studies were:

Field

<0.1 µT

0.1-0.2 µT

0.2-0.4 µT

>0.4 µT

New German study    
Relative risk: 24 hour

1

1.15

1.16

5.81

Relative risk: night-time

1

1.42

2.53

5.53

Pooled German studies:    
Relative risk: 24 hour

1

1.10

1.58

2.48

Relative risk: night-time

1

1.37

3.26

3.44

This prompted a new study to see whether this same "night-time exposure" effect was present in other comparable studies.  The results from the German studies were pooled with the USA, UK and Canadian studies from the Ahlbom pooled analysis.  This new pooled analysis found almost exactly the same relative risks for night-time and 24-hour exposure:

Field

<0.1 µT

0.1-0.2 µT

0.2-0.4 µT

>0.4 µT

Relative risk: 24 hour

1

1.09

1.2

1.98

Relative risk: night-time

1

1.11

1.37

1.93

If there had been higher relative risks for night-time exposure, this could be either because exposure at night-time is more biologically relevant, or because the studies assess night-time exposure (when the child is in the home) more accurately.  But these results "do not support the hypothesis that nighttime measures are more appropriate" (the authors' own words).

Of course, it is still possible to postulate a mechanism whereby night-time exposure could be biologically relevant, and that would not necessarily be contradicted by the epidemiological evidence.  But there is no longer the epidemiological evidence in favour of that idea.

The study abstract

Am J Epidemiol.2007 May 7;


Nighttime Exposure to Electromagnetic Fields and Childhood Leukemia: An Extended Pooled Analysis

Schuz J, Svendsen AL, Linet MS, McBride ML, Roman E, Feychting M, Kheifets L, Kheifets L, Lightfoot T, Mezei G, Simpson J, Ahlbom A.


Institute of Cancer Epidemiology, Danish Cancer Society, Copenhagen, Denmark.

It has been hypothesized that nighttime bedroom measurements of extremely low frequency electromagnetic fields (ELF EMF) may represent a more accurate reflection of exposure and have greater biologic relevance than previously used 24-/48-hour measurements. Accordingly, the authors extended a pooled analysis of case-control studies on ELF EMF exposure and risk of childhood leukemia to examine nighttime residential exposures. Data from four countries (Canada, Germany, the United Kingdom, and the United States) were included in the analysis, comprising 1,842 children diagnosed with leukemia and 3,099 controls (diagnosis dates ranged from 1988 to 1996). The odds ratios for nighttime ELF EMF exposure for categories of 0.1-<0.2 muT, 0.2-<0.4 muT, and >/=0.4 muT as compared with <0.1 muT were 1.11 (95% confidence interval (CI): 0.91, 1.36), 1.37 (95% CI: 0.99, 1.90), and 1.93 (95% CI: 1.11, 3.35), respectively. The fact that these estimates were similar to those derived using 24-/48-hour geometric mean values (odds ratios of 1.09, 1.20, and 1.98, respectively) indicates that the nighttime component cannot, on its own, account for the pattern observed. These results do not support the hypotheses that nighttime measures are more appropriate; hence, the observed association between ELF EMF and childhood leukemia still lacks a plausible explanation.

SCENIHR conclusions on nighttime exposure

SCENIHR conclusions

This issue was considered by SCENIHR in their 2009 report:

"An extension of a pooled analysis of studies on magnetic fields and childhood leukaemia (Ahlbom et al. 2000) showed that focussing on exposure during the night time period gives basically the same results as for exposures over 24 hours (Schüz et al. 2007). This does not support assumptions that exposure during the night is of higher biological relevance or that the restriction to the night time period reduces exposure misclassification."

Full version of tables with confidence intervals

For simplicity, in the tables earlier on this page we gave just the relative risks without the confidence intervals.For completeness, the following table gives the same relative risks together with 95% confidence intervals.

Field

<0.1 µT

0.1-0.2 µT

0.2-0.4 µT

>0.4 µT

Ahlbom pooled analysis        
Relative risk: 24 hour

1

1.08 (0.89,1.31)

1.11 (0.84,1.47)

2.00 (1.27,3.13)

New German study        
Relative risk: 24 hour

1

1.15 (0.73,1.81)

1.16 (0.43,3.11)

5.81 (0.78,43.2)

Relative risk: night-time

1

1.42 (0.90,2.23)

2.53 (0.86,7.46)

5.53 (1.15,26.6)

Pooled German studies:        
Relative risk: 24 hour

1

1.10 (0.75,1.60)

1.58 (0.74,3.36)

2.48 (0.70,8.86)

Relative risk: night-time

1

1.37 (0.94,2.00)

3.26 (1.49,7.11)

3.44 (1.03,11.4)

New pooled analysis:        
Relative risk: 24 hour

1

1.09 (0.89,1.32)

1.2 (0.89,1.63)

1.98 (1.18,3.35)

Relative risk: night-time

1

1.11 (0.91,1.36)

1.37 (0.99,1.90)

1.93 (1.11,3.35)