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Bicycle helmets: review of effectiveness (No.30)

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Section 2: Bicycle helmets standards

Almost all bicycle helmets that are the subject of the studies covered in this report conform to specific national and, very occasionally, international standards. However, it is rare for a study to report on the makes and models of helmets found in the study or the standards to which they conform. Given that most of the papers are based on the injuries that the wearer sustained (or did not sustain) rather than the engineering performance of the helmet, this is not surprising.

How bicycle helmets work

To understand the differences between helmets and the standards with which they conform, it is necessary to have a basic understanding of what helmets are intended to do and what they are not expected to.

Put simply, bicycle helmets (and most other sorts of helmets) aim to reduce the risk of serious injury due to impacts to the head. Serious head injuries can take two forms: skull injuries and brain injuries. While simple fractures to the skull can heal, brain injuries, unlike those to other body regions, do not and can lead to long-term consequences.

Bicycle helmets perform three functions:

  • reducing the deceleration of the skull and hence brain by managing the impact. This is achieved by crushing the soft material incorporated into the helmet;
  • spreading the area over which the forces of the impact reach the skull to prevent forces being concentrated on small areas of the skull; and
  • preventing direct contact between the skull and the impacting object.

These three functions can be achieved by combining the properties of the soft, crushable material that is incorporated into helmets usually referred to as the liner, although it may be the only material of which the helmet is actually made and the outer surface of the helmet, usually called the shell. Historically, helmets had hard shells but now the tendency is for there to be no shell at all or a very thin shell. This leads to lighter helmets that are more acceptable to the wearer. To some extent, the shell was an artefact of one of the tests that the helmet had to pass: dropping the helmet on to a sharp or pointed object (or the dropping of such an object on to the helmet). When accident data are examined, such an impact is very rare and so helmet standards now take this into account, resulting in the lighter helmets.

To work at all, the helmet has to stay on the wearers head during the impact phase. Helmets therefore have retention systems usually a system of chin and neck straps that are tested to ensure that they do not break and that they prevent the helmet rolling off the head when a force is applied upwards at the back of the helmet as can occur when a rider is sliding along the road. Helmets have to work in a variety of climatic conditions: sunlight, containing strong ultraviolet light that can damage plastics over time; rain; heat; and cold. Some plastics become brittle when cold or softer when hot. However, helmets are required to provide adequate protection in all conditions, so the tests to which a helmet is subjected are performed following treatment in these conditions.

The amount of the head that a helmet can protect is driven by the needs of the bicyclist. Ideally, the helmet should provide protection against impacts anywhere on the skull, including the face, but the need for the wearer to see upwards and sideways, hear traffic and be able to tilt their head back when riding because of the seating position limits the extent of coverage significantly. In addition, the need for the head to be reasonably cool necessitates the incorporation of ventilation slots into the helmet.

Other factors that have to be taken into account are the tolerance to impact injury of the human head and the size/weight of the helmet that a rider is willing to use. Research has shown that decelerations of about 250300g are the maximum that can be tolerated by the adult head without leading to irreversible injury. (g is the acceleration due to gravity, approximately 9.8 m/s2). However, young children have lower tolerance to impacts and there are strong arguments for helmets for children having different maximum allowable deceleration, as in the Canadian standard. The thicker the energy absorbing material in the helmet, the better it is able to protect. Helmets are designed to provide a certain level of protection that still allows them to be of a socially acceptable size.

Bicycle helmet standards specify criteria that helmets have to meet when tested in reproducible ways in the laboratory, covering most of the points mentioned above impact absorption over a minimum specified area and under defined environmental conditions, and retention system strength and effectiveness. Design restrictions are usually incorporated into standards to cover peripheral vision and hearing obstruction requirements. Product information is specified through marking, labelling, point of sale information and instructional requirements. Finally, there are usually limitations on the types of materials that can be used to ensure that there are no adverse reactions between the helmet and the skin, sharp edges and points are outlawed internally and externally, and in some cases the total mass of the helmet is limited.

Standards have evolved and changed over time reflecting the state of knowledge of real crashes and the ways in which helmets have failed to provide protection. Given that most of the key requirements in standards are specified in terms of performance in tests, they do not restrict the development and use of new materials nor of the skills of the designer.

Comparisons of standards

This section compares the key requirements of the most common bicycle helmets, namely:

ANSI Z90.4

American National Standards Institute standard Z90.4. One of the first bicycle helmet standards and the basis for many of the others.

ASTM F1447

ASTM International (formerly the American Society for Testing and Materials). Before the CPSC regulation came into force (see below), more than 70 per cent of the bicycle helmets manufactured were certified to this standard. Very similar to the CPSC regulation. Has a certification procedure similar to the Snell system.

Snell B-90S

Specification from the Snell Memorial Foundation, an American nonprofit body. BS-90 is usually recognised as being a specification leading to high quality helmets. Has a rigid certification procedure.

Snell B-95

Also from the Snell memorial Foundation. More demanding than the B-90S specification; some argue that it is too demanding.

CPSC

Regulation enacted by the US Consumer Product Safety Commission in 1998, coming into force in 1999. All helmets sold in the US have to meet its requirements.

BS 6863: 1989

British Standards Institution specification, published in 1989 and later slightly amended. Withdrawn under CEN rules when the CEN standard EN 1078 (see below) was published in 1997.

EN 1078

Standard used by all members of CEN, the European standards-making body. Published in 1997.

EN 1080

Standard used by all members of CEN, the European standards-making body. Published in 1997 to address problems associated with strangulation of children playing while wearing helmets. Intended for helmets for young children.

AS/NZS 2063

Joint Australian and New Zealand standard published in 1996. Noted as a highly respected specification.

CSA-D113.2-M

Canadian Standards Association specification. One of the very few standards that contains specific requirements for helmets for young children.

In Table 2.1, the key features of these standards are presented. As can be seen, in terms of these criteria, the different standards are substantially similar, the main differences being the input energy during the drop tests. Only the Australia/New Zealand and Canadian standards take serious account of the requirements of children by specifying different requirements for helmets tested on smaller headforms.

One standard that is significantly different from the others is EN 1080. This was drafted following a number of fatal strangulations of children playing on playground equipment and elsewhere while wearing helmets complying with EN 1078. In these incidents, the helmeted child became trapped in the playground equipment (through which an unhelmeted head would pass) and the childs weight was supported by the chinstrap. To accommodate this scenario, the standard was written that allowed helmets with a relatively weak retention system to be manufactured so that if such a potential strangulation occurred the retention system would open. When this standard was being drafted, the fear was expressed that such a helmet could come off in a crash, when it is essential that the helmet stays in place in order to provide protection to the head. EN 1080 has no test of the effectiveness of the retention system (the so-called roll-off test).

There is little, if any, research evidence that helmets complying with one standard as opposed to another perform better in protecting the wearer in the event of a crash.

It has to be remembered that before the publication of the CPSC regulation in the USA, none of the standards was mandatory. In Europe, there is no requirement to conform to the CEN standard. In practice, a manufacturer only has to meet the so-called essential safety requirements of the Personal Protective Equipment Directive, with compliance with EN 1078 being one means of achieving this. The certification and enforcement processes are important aspects of this issue; a good standard without an independent certification process or a poor enforcement procedure may allow inadequate helmets on to the market.

Until the Snell B90S standard with its associated certification procedure appeared in the USA, conformity was entirely in the hands of the manufacturer. In the UK, helmets carrying the BSI Kitemark were certified as meeting BS 6863 but the third party certification scheme was not mandatory. Currently, it is trading standards officers in the UK who would confirm compliance with the CEN standard, but this would probably only take place in the event of a dispute or significant helmet failure.

Key points

  • Bicycle helmets aim to reduce the risk of injury due to impacts on the head.
  • Bicycle helmets perform three functions: reduce the deceleration of the skull; spread the area over which the forces of impact apply; and prevent direct impact between the skull and impacting object.
  • A range of different helmet standards have been developed in different countries but they are substantially similar. The main differences relate to the impact energy during the drop tests.
  • Only the Australian/New Zealand and Canadian standards take serious account of the requirements of children, whose tolerances are lower.
  • There is little evidence that helmets of different standards perform better in protecting the wearer.

Table 2.1: Key features of bicycle helmet standards

 

ANSI Z90.4

ASTM F1447

Snell B90S

Snell B95

CPSC

Country of origin

USA

USA

USA

USA

USA

Status

Published 1984. Withdrawn, 1995

Current

Published 1990

Published 1995

Came into force 1999

Anvils

Flat and 50mm radius hemispherical

Flat, 48 mm hemispherical, and kerbstone

Flat, 48 mm hemispherical, and kerbstone

Flat, 48 mm hemispherical, and kerbstone

Flat, 48 mm hemispherical, and kerbstone

Drop apparatus

Twin wire drop rig

Guided free fall

Guided free fall

Guided free fall

Guided free fall

Impact velocity, energy or drop height flat anvil

4.57 m/s

2.0 m (6.2 m/s)

100 J (2.0 m)

110 J (2.2 m) for certification; 100 J (2.0 m) for follow-up testing

6.2 m/s

Drop height other anvils

4.57 m/s

1.2 m (4.8 m/s)

65 J (1.3 m)

72 J (1.5 m) for certification; 65 J (1.3 m) for follow-up testing

4.8 m/s

Impact energy criteria

< 300g

< 300g

< 300g

< 300g

< 300g

Roll-off test

None

Yes

Yes, in 1994 revision

Yes

Yes

Retention system strength

Force applied dynamically. Helmet supported on headform.

Force applied dynamically. Helmet supported on headform.

Force applied dynamically. Helmet supported on headform.

Force applied dynamically. Helmet supported on headform.

Force applied dynamically. Helmet supported on headform.

 

Table 2.1: Key features of bicycle helmet standards (continued)

 

BS 6863

EN 1078

EN 1080

AS/NZS 2063

CSA-D113.2-M

Country of origin

UK

Countries in membership of CEN (EU and EEA)

Countries in membership of CEN (EU and EEA)

Australia and New Zealand

Canada

Status

Published 1989. Withdrawn, 1997

Published, 1997

Published, 1997

Published 1996

Published 1996

Anvils

Flat and kerbstone

Flat and kerbstone

Flat and kerbstone

Flat

Flat and cylindrical (50 mm radius)

Drop apparatus

Twin wire drop rig

Guided free fall

Guided free fall

Twin wire drop rig

Twin wire drop rig

Impact velocity, energy or drop height flat anvil

4.57 4.72 m/s

5.42 5.52 m/s

5.42 5.52 m/s

1.451.80 m.

Ranges from 80 J for largest headform to 34 J for smallest

Drop height other anvils

4.57 4.72 m/s

4.57 4.67 m/s

4.57 4.67 m/s

 

Impact energy criteria

< 300g

< 250g

< 250g

< 300g, < 200 g for 3 ms, < 150g for 6 ms

Level depends on headform size and drop energy. Ranges from 150g for smallest to 250g for largest

Roll-off test

None

Yes

Yes, in 1994 revision

Yes

Yes

Retention system strength

Force applied dynamically. Helmet supported on headform.

Force applied dynamically. Helmet supported on headform.

Force applied gradually until helmet releases from headform. Helmet supported on headform.

Force applied statically

Force applied dynamically

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