Two different types of cellular phones were included in the study: the handheld model which has the antenna in the hand piece in close proximity to the head during a call, and the nonhandheld model, mainly car telephones. Users of nonhandheld model are considered as unexposed because the antenna is not part of the hand set.
|Reference group 1||nonhandheld telephones, calling time: median 1.5 min/day|
|Group 2||handheld telephones, calling time: < 2 min/day, median 0.8 min/day|
|Group 3||handheld telephones, calling time: ≥ 2 min/day, median 5.0 min/day|
|Reference group 4||nonhandheld telephones, length of service: median 2.1 years|
|Group 5||handheld telephones, length of service: ≤ 3 years, median 1.6 years|
|Group 6||handheld telephones, length of service: > 3 years, median 3.8 years|
The only category of cause of death for which there was indication of increasing risk with increasing minutes of cellular telephone use was "motor vehicle collisions". Similar results were found for number of calls per day. The distinction between handheld and nonhandheld telephones did not apply since both telephone types might interfere with driving.
To investigate the relation between cellular telephone use and risk of fatal motor vehicle collisions, the authors combined handheld and nonhandheld cellular telephone users and looked for trends. They found increasing risk of mortality with increasing telephone use, but an inverse trend with number of years of service.
Because the analyses were based on billing data, some information were missing: which calls were placed from a motor vehicle, whether a cellular telephone was used immediately preceding the fatal motor vehicle crash, how many hours individuals drove and the relation between driving hours and telephone hours.