A Survey on the Usage of Hard Hats in Hong Kong Construction Sites,
and a Study on the Protective Functions of Safety Helmets
Falling object is one of the main reasons responsible for the deaths of many construction
workers in Hong Kong. In order to improve the occupational safety of the construction
industry, enhance efficiency and protect the lives of construction employees, the
Occupational Safety and Health Council has commissioned and sponsored Professor
Tongxi Yu, Dr. Jingshen Wu and Mr. Xiaowei Chen of the Department of Mechanical
Engineering of the Hong Kong University of Science and Technology, to conduct a
thorough study on the protection functions of hard hats. The study began in April 1997 and
completed in March 1999.
An extensive survey in the market and construction sites has been conducted. Through
interviews, mails and faxes, researchers have surveyed hard hats’ manufacturers, looked
into the standard of craftsmanship, the basis of purchase, replacement procedures, incidents
situation, some of the most popular brands in the territory, and some prevalent defects, and
they found :
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The safety standards adopted in the Hong Kong construction industry vary, but
ISO 3873-1977 and the corresponding BSI are the main standards used.
The construction sites interviewed used more than ten brands of hard hats, our
research targets the three most common brands which we will name A, B and C.
They come from UK and Australia.
All hard hats are made of plastics, including High Density Polyethylene (HDPE),
Low Density Polyethylene (LDPE), Acrylonitrile-Butadiene-Styrene (ABS) and
Polycarbonate (PC).
Prices and safety standards are the major considerations of purchase.
Under no damage condition, hard hats used in construction sites are generally
replaced every two to three years.
The researchers also discovered a few characteristics of the Hong Kong construction
industry. First, Hong Kong has an extraordinarily large number of high-rises, making
falling objects an even more dangerous weapon, the current international requirements on
impact resistance of hard hats may not be adequate for Hong Kong’s condition. Second,
most hard hats currently in use are made of plastics, the hot and humid weather condition
in Hong Kong may drastically lower their impact resistance function and shorten their
service lives, that also means the international standards may not be adequate here.
Although the number of incidents caused by falling objects are not great, but they are
always fatal when happened, the researchers notice that workers generally have little
knowledge on hard hats, training is needed in this area.
Impact tests on hard hats
Seven different sets of impact tests have been performed, they are : central impact test,
off-centre impact test, penetration test, impact test of hard hat with minor damage, impact
test of weathering aged hard hat, time delay test and neck injury test. The following is a
brief on part of the tests.
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Central impact test
The impact point was at the top-centre of the hard hat samples. According to ISO
3873-1977 safety standard, when the Maximum Transmitted Force (MTF) is higher than
5000N, or when the hard hat cracks after 50J impact, the hat will lose its function of
protecting the head. In the test, both A and B hats, even though their structure stay intact,
have failed the protection function as they transmit more than 7000 N and 8000N
respectively under 100J impact energy. C’s MTF stays under 3000J, which is a better
brand. When the impact energy goes up to 150 J, all three show cracks or stress whitening,
reflecting serious damage in the hat and replacement is needed immediately.
Suspension is a critical part of a hard hat. It delays and reduces the peak force transmitted
to the head and neck. A regular check on the suspension is strongly recommended on
construction sites.
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Penetration test
Six samples for each brand have been tested. A hard hat is considered inadequate if a
three- kilogram conical striker dropped from a height of 1 meter contacts the head model
upon impact. All three passed the test under the ISO 3837-1977 standard.
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Impact test on hard hats with minor damage
Minor damages on the hard shell generally do not result in bigger damages after impact,
but if there were small cracks on the suspensions, almost all safety hats became completely
damaged even with a 50J impact. This further demonstrates the importance of the
suspension.
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Impact test of aged hard hats resulting from weathering
Extreme heat and high humidity could catalyse the ageing process of hard hats.
Researchers conducted impact test in the laboratory after treated hard hats in 50 degree
Celsius oven for four hours, and submerged another set of samples in water for four hours.
All samples showed insignificant changes after the tests.
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Tests of used hard hats from construction sites
All used brand A hard hats, including those which have been used for two years, passed
the 50J impact test; but when impact energy slightly increased, the MTF of those used for
1.5 to two years rose to nearly 5000N (i.e. the ISO threshold). Researchers point out the
weathering ageing factor is significant, and suggest more frequent replacement of hats in
construction sites. For those sites with higher risk of falling objects, more protection
measures should be employed.
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Neck injury test
Neck injury has never been considered in the hard hats’ standard. This study, for the first
time in Hong Kong, looked into this factor to see how much the neck is protected by the
safety helmet.
The test showed that brand A hats can continuously provide reasonably good protection for
at least two years if the impact level is low and there is no pre-existing damage on the hat,
but when impact energy was slightly raised to 60J, the MTF rose near the safety threshold.
That implies that brand A hats have a very low safety margin. Considering the buildings in
Hong Kong’s construction sites are usually very high and a small falling object can easily
have an impact energy higher than 50J, it is suggested that brand A hard hats should not be
considered as the first choice.
Tests on hard hat materials
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Tests of the hot water-treated materials: all materials tested showed deterioration in
ductility and toughness, and all the hard hat materials were embrittled after the
temperature treatment.
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Mechanical tests of the environmental chamber treated materials: 48 samples of the
three brands were conducted an accelerated environmental treatment with Q-U-V,
which simulated the effect of sunshine before tests were performed The results
found that brand C hard hat’s ABS material is very sensitive to weathering.
Hence, it is suggested that frequent replacement is needed if brand C is used.
Conclusions:
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All new hard hats in Hong Kong’s market can successfully pass the standard
impact tests; their safety factor ranges between 2 to 3. The impact strength of the
hard hats declines significantly after 2-3 years of service, and will not be able to
provide the level of protection as expected.
The impact resistance of hard hats depends on their materials and structure.
Generally, the hard shell and the suspension both deform to absorb energy from
impact and to protect the head. The suspension, especially, has an essential
function that cannot be neglected.
The hard shell of the safety helmet may lose its function by denting, fracture or
whitening; while the detachment or fracture of the suspension can also cause a
safety helmet to lose its protective function completely and result in fatal injuries.
That is why more care should be put to ensure suspensions are of good quality and
in good working condition.
Weathering conditions, such as high UV light, high temperature and high humidity,
have remarkable influence on plastic hard hats, making it brittle and hence
reducing its impact strength.
Neck injuries caused by impact on hard hats were assessed for the first time in
Hong Kong. Central impact and off-centre impact on the hat may cause cervical
spine injury, as a result of excessive pressure and/or bending. The test results show
that for certain hard hats brands (such as A), a standard impact of 50J is sometimes
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enough to cause cervical spine injury.
In the table below we have summarised the test results of the three hard hat brands
most popularly used in the construction sites which we interviewed. Brand C has
the highest performance in terms of impact resistance, but is most sensitive to
weathering. If brand C is chosen it should be replaced frequently. Brand A, despite
passing standard tests, has the lowest safety factor amongst the three brands. In
most tests, brand A can only just reach the safety threshold, its safety factor is
comparatively low, but unfortunately, it is the most popular brand used in the
construction sites we have interviewed.
The comparison of the three brands
A
B
C
Weight (kg)
0.31
0.32
0.44
Hard shell
HDPE
HDPE
ABS
Suspension
material
LDPE
LDPE
HDPE
Hardness
Low
Adequate
High
Safety factor for
preventing hard
shell damage
2.0
3.0
3.0
Safety margin
for head
protection
Low
Low
High
Impact
resistance
Good
Good
Excellent
Penetration
resistance
Low
High
High
material
Decay of force
pulse
Low
High
High
Energy
absorbing
capacity
Acceptable
Good
Excellent
Suspension
function
Not very good
Good
No very good
Influence of
temperature,
Relatively
insignificant
Relatively
insignificant
Significant
High
Low
humidity and
UV
Risk of head and High
neck injury
Recommendations
1. What kind of hard hat standard should be used in Hong Kong?
Although the ISO 3873 standard is universally accepted, some standards are
stricter than others in terms of replacement cycle and safety inspection; also some
standards are set and tested according to Asian head characteristics, such as the GB
(Chinese international). Given the geographical and weather conditions in Hong
Kong, users should consider these standards when choosing safety helmets.
2. The safety factor of hard hats
The international safety standards used in Hong Kong are ISO, ANSI, BSI, GB and
JIS etc. They require the hard shell to stand an impact energy of 50J. However,
considering the special conditions of Hong Kong’s construction sites, the impact
resistance should be set higher. The safety factors of the hard hats tested are 2 to 3,
therefore, users should consider a safety factor of hard hats of not less than 2.0 on
top of the standard 50J impact energy when buying safety helmet.
3. Hard hat materials
Under the hot and humid weather conditions in Hong Kong, the plastic materials of
hard hats will age quickly; the ABS material, for example, ages more obviously.
Brand C, although having the best protection against impact, is one that is worst
affected by weathering in reducing its safety level. Therefore, construction sites’
foreman and workers should keep in mind the manufacturing materials of the hard
hats and replace used hard hats regularly.
4. Replacement of hard hats
Most safety standards do not state a replacement time for hard hats. The tests of brand A
show its protection from impact force deteriorates after two years, and cannot provide the
protection as expected. It is therefore recommended that in Hong Kong construction sites,
maximum service time of plastic hard hats in should not exceed 2 years.
5. Safety training and replacement plan
The Factories & Industrial Undertakings (construction site)(safety) regulations require the
contractors to provide safety helmets to every worker employed in construction sites, and
to implement reasonable procedures, to ensure no one can stay inside constructions sites
without wearing proper safety helmet. The general duties in the Occupational Safety &
Health Ordinance also require the employers to provide safety training to employees.
According to our observation, most construction workers lack the training of the safe
handling of hard hats, and they do not have enough knowledge on the maintenance and
regular replacement of hard hats.
6. The hard shell and suspension of hard hats sustain frequent impacts from small falling
objects and minor bumps and scratches, which may affect their reliability and service life.
It is therefore necessary to remind users to carefully check the hard shell and suspension
every time before use.
Fig 1. Dynatup 8250 instrumented drop-weight impact tester
Fig 2. Headform and strain gauges used in the tests
Fig 3. Brand B hard hat after 150J over-loaded impact. A crack can be seen
Fig 4. Suspension breakage of a Brand C hard hat after an over-loaded impact (100J)
Fig 5. Peak force vs. service time of Brand A hard hats
Fig 6. Peak load at impact point vs. service-time for the used Brand A hard hats under 50J
and 60J impact tests
Fig 7. Peak load at neck vs. service-time for the used Brand A hard hats