Residential and occupational exposure to sunlight and mortality from non-Hodgkin's lymphoma: composite (threefold) case-control study.

Introduction

Incidence and death rates for non-Hodgkin's lymphoma have risen rapidly in recent decades throughout the world.[1] Neither changes in diagnostic practices nor known and suspected risk factors seem to account fully for the increases.[2] Several investigators have hypothesised that increased exposure to sunlight may have contributed to the rising rates of non-Hodgkin's lymphoma[3-8] They note that melanoma and other skin cancer rates have also risen rapidly worldwide,[4] that non-Hodgkin's lymphoma and skin cancer are associated in individual patients,4 6 and that ultraviolet stimulation is experimentally immunosuppressive.[3 7] Ecological studies that have explored this hypothesis have had conflicting results.[3 7-9] None, however, examined sources of sunlight exposure other than residence.

We conducted a population based case-control study of non-Hodgkin's lymphoma mortality in the United States. As an improvement over ecological studies, we assessed potential sunlight exposure from both the occupational and the residential records on the death certificate. The findings for non-Hodgkin's lymphoma were compared with findings for melanoma and non-melanotic skin cancer, two diseases regarded as causally linked to sunlight exposure.[10]

Methods

Since 1984 the American National Cancer Institute and the National Institute for Occupational Safety and Health have supported the creation of a 24 state mortality database, which includes coding of occupation and other information from death certificates, as described by Figgs et al.[11] Cases included all deaths between 1984 and 1991 from non-Hodgkin's lymphoma (International Classification of Diseases, ninth revision (ICD-9), codes 200 and 202 (excluding 202.2-202.6)), melanoma (code 172), and non-melanotic skin cancer (codes 173,154.3, and 187.7) in people aged 20 or over identified as white or African American. Two controls per case were selected from non-cancer deaths in the database and matched for frequency by sex, race, and five year age group.

Potential sunlight exposure was assessed by residence and usual occupation recorded on the death certificate. The annual mean daily solar radiation (which includes ultraviolet and visible light) for the state of residence at death and state at birth were obtained from United States Weather Bureau data.[12] Each state was characterised at one of three levels based on the predominant solar radiation contour crossing the state. The 24 states reflected all regions of the country (see table 1).

Table 1 Characteristics of non-Hodgkin's lymphoma, melanoma, and non-melanotic skin cancer cases and controls.([dagger]) Data expressed as numbers (percentages) of subjects

 
                                        Non-Hodgkin's lymphoma 
 
                                        Cases           Controls 
                                   (n=33 407; 34%)  (n=65 843; 66%) 
Age (years): 
  20-44                                2 320 (7)       4 413 (7) 
  45-54                                2 587 (8)       4 994 (3) 
  55-64                                5 646 (17)      11 089 (17) 
  65-74                                9885 (30)       19 590 (30) 
  [is greater than or equal to]75      12 969 (39)     25 757 (39) 
Sex: 
  Female                               16 609 (50)     32 822 (50) 
  Male                                 16 798 (50)     33 021 (50) 
Race 
  Black                                 1 818 (5)       3 535 (5) 
  White                                31 589 (95)     62 308 (95) 
Pigmentation: 
  Fair([double dagger])                 1 806 (5)       3 782 (6) 
  Other white                          29 783 (89)     58 526 (89) 
  Black                                 1 818 (5)       3 535 (5) 
  Residence([sections]) 
  Low sun                              11 553 (35)     21 244 (32) 
  Moderate sun                         13 529 (41)     26 334 (40) 
  High sun                              8 325 (25)     18 265 (28) 
Birthplace([parallel]): 
  Low sun                              11 000 (33)     20 496 (31) 
  Moderate sun                         14 290 (14)     27 861 (42) 
  High sun                              6 863 (21)     15 245 (23) 
Occupation: 
  Indoor                               17 054 (51)     31 125 (47) 
  Mixed                                14 081 (42)     29 502 (45) 
  Outdoor (non-farmer)                    867 (3)       2 302 (4) 
  Farmer                                1 405 (4)       2 914 (4) 
Socioeconomic status([paragraph]) 
  1 Low                                 3 653 (11)      8 917 (14) 
  2                                     4 889 (15)     11 055 (17) 
  3                                     9 771 (29)     17 815 (27) 
  4                                     5 177 (15)      8 001 (12) 
  5 High                                1 449 (4)       1 954 (3) 
 
                                               Melanoma 
 
                                           Cases     Controls 
                                   (n=12 156; 34%)   (n=23 845; 66%) 
Age (years): 
  20-44                                2 290 (19)      4 361 (18) 
  45-54                                1 753 …