OK there might be a few cases where a strap might catch on something and add to injury but in general I can`t see that wearing a helmet would add any danger to my riding.
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There does appear to be some evidence that helmet's increase risk as touched on in the report below.
Risk compensation
Rodgers re-analysed data which supposedly showed helmets to be effective; he found data errors and methodological weaknesses so serious that in fact the data showed "bicycle-related fatalities are positively and significantly associated with increased helmet use".[67] A range of theories have been proposed to explain why helmet use might indirectly translate into more or worse accidents. In short, the analysis of helmet effectiveness is confounded by changes in human behaviour apparently induced by the presence of protective headgear.
Under the risk compensation theory, helmeted cyclists may be expected to ride less carefully; this is supported by evidence for other road safety interventions such as seat belts and anti-lock braking systems.[68][69] There is some evidence for risk compensation by children in relation to safety equipment.[70] Anecdotally, many riders report feeling safer with a helmet: "When I wear it, I feel safe..."[54] One researcher randomized his helmet use over a year of commuting to work and found that he rode slightly faster with a helmet.[71]
Motorists may also alter their behavior toward helmeted cyclists. One small study from England found that vehicles passed a helmeted cyclist with measurably less clearance (8.5 cm) than that given to the same cyclist unhelmeted (out of an average total passing distance of 1.2 to 1.3 metres).[72]
Rotational injury
It has been suggested that the major causes of permanent intellectual disablement and death after head injury may be torsional forces leading to diffuse axonal injury (DAI), a form of injury which usual helmets cannot mitigate and may make worse.[73] Helmets may increase the torsional forces by increasing the distance from the centre of the spine to the outside of the helmet, compared to the distance to the scalp without a helmet: "Bicycle helmet crash simulation experiments carried out as part of this project indicated very high rotational accelerations for a fall over the handlebars at 45 km/h. The rotational accelerations were found to be 30 percent higher than those found in similar tests using a full face polymer motorcycle helmet."[74] A 1991 study by Hodgson, in which bicycle helmets were tested for ease of skidding, found that adding facial protection to a standard bicycle helmet (in effect making the helmet full-face) brought the benefit of reduced twisting forces on the brain.[75]
Accidental hanging by helmet straps
There are occasional cases of young children playing (on or near bunk beds, trees, clothes lines, play equipment etc.) suffering death or severe brain damage as a result of hanging by the straps of their bicycle helmets.[79][80][81][82][83][84][85][86][87] To avoid rare serious accidents, a bicycle helmet should not be worn by children when climbing or using play equipment such as trampolines while unsupervised