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EVectiveness of toughened glassware in terms of
reducing injury in bars: a randomised controlled
trial
Alison L Warburton, Jonathan P Shepherd
Abstract
Objective—To evaluate the eVectiveness,
in terms of injury prevention, of tough-
ened pint glassware in bars.
Design—Randomised controlled trial.
Setting—A random sample of 57 bars in
South Wales, West Midlands, and West of
England.
Subjects—A total of 1229 bar workers.
Intervention—Complete replacement of
pint glasses with annealed (control) or
toughened (intervention) glassware.
Main outcome measures—Bar staV inju-
ries recorded monthly: number, site, and
severity (lifestyle impact; treatment need)
of injuries.
Results—Ninety eight bar staV experienced
115 injuries: 43 in the control group, 72 in
the intervention group. Adjusting for peo-
ple at risk gave a relative risk (RR) of 1.48
(confidence interval (CI) 1.02 to 2.15).
Similarly, adjusting for hours worked gav e
RR 1.57 (CI 1.08 to 2.29). Thus, injury rate
was 60% higher in the intervention group
(p<0.05), with no significant diVerence in
severity. Most were hand injuries requiring
first aid. Injuries tended to occur simulta-
neously in more than one body part in the
intervention group, reportedly caused by
spontaneous disintegration of toughened
glassw are. Impact resistance testing
show ed the energy required to break an-
nealed glass (1.8 ± 0.2 J) was greater than
that for toughened glass (1.4 ± 0.2 J), though
the diVerence was not significant.
Conclusions—Glass with lower impact
resistance caused more injuries. “Tough-
ened” glassware had lower impact resist-
ance. Standards for toughening need to be
developed.
(Injury Prevention 2000;6:36–40)
Keywords: toughened glass; bar workers; quality control
Bar glassware is responsible for about 10% of
assault injuries that present to UK emergency
units
12
and these usually lead to permanent,
disfiguring facial scars.
3
In 75% of cases, the
one pint “Nonic” glass has been implicated.
3
The Nonic is a straight sided one pint glass
with a bulbous portion towards the rim.
Toughened glass is produced by a process that
involves rapid cooling of the glass after its initial
formation, thereby producing a compressive
outer skin that holds the outer layer together.
Because all glass tends to break at the site of
microcracks and flaws, toughening prevents
breakage in the outer layer by holding together
such flaws and preventing their propagation. In
contrast, annealed glass is cooled slowly to
eliminate all internal stresses, and propagation
of flaws and cracks is more likely.
Shepherd et al demonstrated two important
advantages of toughened bar glassware.
4
First,
new toughened glasses were six times more
impact resistant than annealed glasses of the
same design. Second, when toughened glasses
did fail they tended to disintegrate into small
“sugar lump” cubes, similar to those resulting
from failure of a toughened car windscreen. In
addition, toughened glasses were twice as
impact resistant than annealed glasses after
being subject to simulated wear and tear. How-
ever, manufacturers of non-toughened glass-
ware have claimed that spontaneous disintegra-
tion of toughened glassware can cause injury.
5
These findings have led, according to the
industry, to a switch from annealed to tough-
ened glassware in UK bars licensed to sell
alcohol. This has been on the assumption that
toughened glassware reduces the likelihood of
assault injury. However, to date, toughened bar
glassware has not been subject to field trials to
determine whether its use does prevent injury,
both intentional and unintentional.
Unfortunately, reliable and quantifiable data
on assault injury in bars is not available, but by
focusing on occupational injuries incurred by
bar staV it is possible to measure the eVective-
ness of toughened glass in terms of reducing
unintentional injury.
Bar workers often sustain cuts from bar glass-
ware, many of which involve deep structures
such as ligaments, and are disabling.
6
A study on
the risk of occupational glass injury in bar staV
revealed that 41% of bar workers in South Wales
have sustained bar glass injury.
7
Fifteen per cent
of these had sustained five or more separate
“sharp” injuries from broken glassware. After
13% of incidents, treatment had been sought in
an emergency unit. Another study found that
74% of bar staV reported lacerations from
broken glassware at work, and of these, 18%
were injured on more than 10 occasions.
8
In
addition, most of those questioned had had
some contact with body fluids such as blood,
vomit, urine, and faeces, increasing the risk of
hepatitis B.
8
Straight sided one pint glasses were
responsible for two thirds of all injuries, usually
during stacking and washing.
Taken together this evidence suggests that
replacement of annealed bar glassware with
toughened glassware reduces the risks of
injury, both unintentional and intentional (see
Injury Prevention 2000;6:36–4036
Violence Research
Group, Department of
Oral Surgery Medicine
and Pathology, Dental
School, University of
Wales College of
Medicine, Heath Park,
CardiV CF14 4XY, UK
A L Warburton
J P Shepherd
Correspondence to:
Professor Shepherd,
Professor of Oral and
Maxillofacial Surgery
Shepherd
9
). Therefore, the study reported here
tested the hypothesis that toughened glassware
is safer than annealed glassware in the bar
environment.
Methods
PROTOCOL
From previous data, it was calculated that over
a six month period, 26% of bar workers were
injured on one or more occasions with one pint
glasses. Thus, a suYcient number of bars to
yield 400 bar staV in two groups was required:
200 in a group using only annealed glassware
(control group) and 200 in a group using only
toughened glassware (intervention group).
This assumes a data collection period of six
months. For the use of the toughened glass to
be worthwhile, the risk needed to be halved so
that one in eight or 25 in the intervention group
would sustain injury. If a reduction of risk of
this size occurs, there is an 80% chance of
detecting it using a statistical test at the
conventional (5%) level. Assuming that on
average each bar provided data from four bar
staV, a minimum of 100 bars, 50 in each group,
were required.
Bars were initially identified from a national
database of licensed premises, through trade
press listings, and local telephone directories in
CardiV, Gwent, Shropshire, StaVordshire,
Stoke-on-Trent, West Midlands, and Bristol.
Recruitment was most eVective by letter. These
letters explained the aims of the trial, and set
out the criteria for eligibility. For inclusion, the
bar had to satisfy six criteria: (1) be able to use
20 oz government stamped one pint Nonic
glasses; (2) be able to use both annealed and
toughened glasses (that is no special conditions
were attached to their drinks license); (3)
cooperate with the researchers by completing
monthly questionnaires (all bar staV); (4) not
to use any other pint glasses other than those
provided during the trial; (5) to have at least
four participating bar staV; and (6) to have
reported problems with glass related injuries in
the previous year.
For those responding to recruitment adver-
tising, a date was arranged for an interview
with the bar manager/landlord. At interview,
managers/landlords were briefed on the aims of
the trial, and their role explained in detail.
They then completed an initial structured
questionnaire designed to ascertain the extent
of glass related injuries and the exact nature
and type of bar operated.
Recruitment was continued until the
number of eligible bars was suYcient to yield at
least 600 members of staV. No record exists of
precise numbers of bars approached, but this
number exceeded 700.
ASSIGNMENT
The unit of randomisation was the bar, as staV
numbers were expected to vary. Managers of 53
of the 60 eligible bars agreed to participate in the
trial (see fig 1) and these were randomly
assigned. The 23 bars in the control group were
assigned annealed glassware, and the 30 in the
intervention group were assigned toughened
glassware. Many types of one pint capacity glass
are available, but in this study only the Nonic
design was used. Each bar replaced its entire
stock of pint glasses with new glasses supplied by
the researchers, and throughout the trial only
used pint glasses supplied by the researchers.
Both types of glass (annealed and intervention)
were from the same manufacturer and obtained
from the same supplier. It was important to use
new glasses because wear and tear is known to
decrease impact resistance.
10
Injuries were recorded using questionnaires
distributed monthly via bar managers/
landlords to all staV working in the bar. Precise
numbers of questionnaires distributed are not
available, but each bar manager/landlord was
provided with excess numbers. Thus, response
rates of staV are not known precisely.
At the end of the trial, all bar workers were
asked to complete an additional questionnaire
concerning their opinions of the glasses: their
perceptions of durability, strength, and break-
age properties.
MASKING
Neither landlords nor their staVs were in-
formed of the type of glass they were using and
neither they nor the researchers knew which of
the glasses was more impact resistant (double
blinding). Although the packaging did reveal
the identity of the glasses, most bar workers
were only aware they were testing a “new” type
of glass.
Figure 1 Progress through the various stages of the trial, in terms of bars participating
(staV numbers varied on a monthly basis, hence only final numbers are included).
Random-
isation
Received annealed (control)
glasses as allocated (n = 23)
Did not receive annealed (control)
glasses as allocated (n = 4)
Received toughened (intervention)
glasses as allocated (n = 30)
Did not receive toughened
(intervention) glasses as allocated
(n = 0)
Followed up (n = 23), on monthly
basis, for 6 month period
Completed trial (n = 14)
Completed questionnaires (n = 576)
Completed trial (n = 16)
Completed questionnaires (n = 653)
Followed up (n = 30), on monthly
basis, for 6 month period
Bars lost to follow ups
Reasons: withdrawal, change of
manager, closure, and delivery
failure (wrong glasses)
Month 1 (n = 3)
Month 2 (n = 3)
Month 3 (n = 0)
Month 4 (n = 0)
Month 5 (n = 1)
Month 6 (n = 2)
Bars lost to follow ups
Reasons: withdrawal, change of
manager, closure, and delivery
failure (wrong glasses)
Month 1 (n = 3)
Month 2 (n = 2)
Month 3 (n = 0)
Month 4 (n = 0)
Month 5 (n = 3)
Month 6 (n = 6)
Not randomised (n = 3)
Reason: withdrew
No of bars eligible (n = 60)
Toughened glassware and injury in bars 37
IMPACT RESISTANCE
Toughened glasses have been reported to
disintegrate spontaneously,
5
and it became evi-
dent as the trial progressed that this was a fre-
quent occurrence with the intervention glasses.
To attempt to explain this, the impact resist-
ance of the two glass types was assessed using
previously described methodology.
4
New trial
glasses, selected at random, were tested in
batches of 10, using a Zwick 5102 pendulum
impact tester in a materials laboratory. The test
rig was set so that the pendulum struck the
glass 1.5 cm below the rim, at a maximum
value of 4 J. For comparison, toughened pint
Nonic glasses (n = 10) from an alternative
manufacturer were also tested.
Results
Some bars were lost to follow up (see fig 1) and
for several reasons, mainly closure. Of those
continuing in the trial, landlords were pursued
until as many of their staV as possible had
completed and returned questionnaires. Some
landlords seemed uncertain as to how many
staV they employed, and some bar workers
either refused to complete the questionnaires,
“forgot” to return them, or were unavailable.
A total of 1229 questionnaires were com-
pleted and returned by bar staV (782 females
and 447 males) over the six month period. The
mean age of staV was 32 years, and the mean
length of service (in the trial bar) was 49
months.
Ninety eight members of staV experienced
115 injury events (see table 1). Of these, 101
were hand injuries, 91 required first aid in the
work place, and four required sutures. The
majority of staV injured did not require time oV
work, and described their injury as only a
minor inconvenience. Nine injuries (8%)
aVected lifestyle severely, but only three
injuries resulted in needing time oV work. One
injury was reportedly sustained in an assault.
There was no significant diVerence regard-
ing severity of injury between the two groups
(see table 2) except that in the toughened
(intervention) group injuries tended to occur
simultaneously in more than one body part.
This is consistent with injuries caused by glass
disintegrating “spontaneously”, that is without
any specific cause, often leading to wide
dispersal of fragments.
ANALYSIS
Are there significantly fewer injuries in the
intervention group, when the number of staV
and number of exposure hours are taken into
account?
Either the number of bar staV injured or the
number of injuries can be analysed, the former
being more appropriate, because the statistical
model assumes that diVerent responses are
independent. Out of 1229 bar staV, 98 experi-
enced one or more injuries, and a total of 115
injury events occurred. Whichever outcome is
chosen, an adjustment must be made for either
the number of bar staV in the group, or the total
number of hours worked over the six month
period.
A method to calculate the relative risk (RR)
and its confidence interval (CI), must address
the proportions (for example 36/576) and
event rates (for example 36/71738) in a similar
manner. The ratio of number of bar staV
injured in the intervention group to number
injured in the control group is 1.72—which can
be calculated either as 62/36 or as 0.63/
(1−0.63). In the same way, a 95% CI for this
ratio is 0.53/(1−0.53) to 0.72/(1−0.72), that is
1.15 to 2.59.
11
This appears to indicate that,
even at a conservative estimate, there is nearly
15% greater risk of injury in the intervention
group. Adjusting for the number of bar staV
gives a final estimated RR of 1.52 (CI 1.01 to
2.28). Alternatively, adjusting for the ratio of
the total hours worked gives an estimated RR
of 1.62 (CI 1.07 to 2.43). Analysing the total
number of injury events (starting with an event
ratio of 72/115 = 0.63, CI 0.54 to 0.71) and
adjusting for bar staV at risk, gives RR 1.48,
and CI 1.02 to 2.15. Alternatively, if the
adjustment is for hours worked a RR of 1.57,
with CI 1.08 to 2.29 results. This tells us that
the injury rate appears to be about 60% higher
in the intervention group. Bearing in mind the
limited sample size, the true statistical “disad-
vantage” of the toughened glass could be just a
few per cent more injuries, or could be a dou-
bling or more. However, the CI excludes the
null hypothesis that the two glasses are equally
safe, that is, there was a diVerence, significant
at p<0.05, in favour of the annealed glass.
IMPACT RESISTANCE
In terms of the energy required to break the
glasses, annealed (control) glasses had a higher
impact resistance than the intervention glasses,
although this was not significant (p=0.35).
Three of the intervention glasses remained
intact after two attempts, whereas all the
annealed glasses were broken. However, in
comparison, all toughened Nonics from an
alternative source withstood the impact. Mean
Table 1 Occupational injuries to bar staV caused by one pint glasses
Annealed glass
(control g roup)
Toughened glass
(intervention group)
No of injuries 43 72
No of people injured 36 62
Total No of people in group 576 653
Total No of hours worked* 71 738 76 356
*Most landlords noted their hours of work as greater than 80 per week. This was assumed to relate
to hours in the trade, as opposed to hours working the bar. Therefore, exposure times were scaled
down—based on average opening hours, and a six day week. For those not recording their hours
of work, the mean number of hours worked by all “ordinary” bar staV was used.
Table 2 Breakdown of injury details by group; results are number (%)
Annealed glass
(control g roup)
Toughened glass
(intervention group)
Site of injury
Hand area (hand, finger, thumb, palm, wrist) 37 (86) 64 (89)
Other 7 (14) 8 (11)
Place treatment sought
First aid 32 (74) 59 (82)
Other 11 (25) 13 (18)
Type of treatment required
Other 33 (76) 52 (72)
Sutures 0 4 (6)
None/not recorded 10 (23) 16 (22)
EVect on lifestyle
Not much 30 (70) 50 (69)
Great eVect 1 (2) 8 (11)
38 Warburton, Shepherd
impact resistance of the glasses tested is
tabulated in table 3.
Consistent with previous studies, annealed
glasses broke into sharp pieces, often leaving
the base intact and potentially dangerous,
whereas the toughened glass disintegrated into
numerous small cuboid fragments with rela-
tively blunt edges.
OPINIONS
All bars in the intervention group reported, at
one time or another, that they were having
problems with glasses disintegrating spontane-
ously. Mostly the glasses would disintegrate
while on the shelf, but occasionally they were
filled and either on the bar or on a table. Forty
four per cent of landlords reported that at least
one brand of new glass disintegrated as soon as
it was taken from the box.
In the intervention group 52% of staV said the
trial glasses were “better” than their previous
glasses; 55% said they were stronger and 60%
said they lasted longer. Forty five per cent
reported a problem with the “new” glasses,
mostly the tendency of toughened glass to disin-
tegrate spontaneously. In addition, the majority
(63%) of the breakage of toughened glass
occurred while the glass was on the shelf—but
not all saw this as a problem per se. Many publi-
cans were willing to accept that toughened glass
is safer and causes fewer injuries, although they
were often irritated by the inconvenience of
clearing many small cubes of glass.
In the control group, 68% agreed the trial
glasses were better and stronger than previous
glasses, and 96% reported no problems. Of
interest is that breakage in this group occurred
in all locations except on storage shelves.
StaV were reasonably well informed of the
characteristics of toughened glass, with 46%
perceiving toughened glass as harder to break;
91% agreed that toughened glass was a good
idea in principle.
Discussion
This trial is the first to evaluate the eVective-
ness of “toughened” glass in terms of injury
prevention. Previous laboratory research has
demonstrated that toughened glassware is
more impact resistant and stronger than
annealed glassware,
4
leading to the hypothesis
that toughened glass is safer than annealed
glass. However, in the trial, the unintentional
injury rate due to toughened (intervention)
glass was 60% higher than that due to annealed
(control) glass. It had been assumed that the
intervention glasses were “toughened” satis-
factorily, as they were widely available to the
licensed trade, and that precise quality control
was exercised during manufacture. However,
laboratory testing, performed to explain the
frequent failure of the intervention glasses in
bars showed that in terms of impact resistance
these particular glasses were less tough than
annealed glasses. In fact, mean impact resist-
ance was lower for these “toughened” glasses.
In comparison, toughened glasses from an-
other manufacturer withstood much greater
impact. This shows that although toughened
products are widely available, they are tough-
ened to widely diVering degrees. Thus if
toughened glass is to be marketed as such then
standards need to be set to ensure that tough-
ened products are what they claim to be.
Glasses should then carry an indication, such
as a UK “kite mark”, of conformity with
manufacturing standards.
There were also problems with the glasses in
the intervention group disintegrating spontane-
ously causing minor superficial injuries. These
injuries were caused either when workers were
clearing away fragments, and less frequently, by
being showered with disintegrating fragments.
Consistent with this was that injuries in the
intervention group often occurred simultane-
ously in more than one anatomical region. Fur-
thermore, most of the intervention glasses
reportedly disintegrated on the shelf, that is
while not in use. By contrast, the annealed
glasses failed in day to day usage, for example,
during collection and cleaning. Once the prob-
lem of spontaneously disintegrating glass has
been addressed, by improving standards of
toughening, most of these injuries would un-
doubtedly be prevented.
Those bars that had used toughened prod-
ucts before reported previous experiences of
the spontaneous disintegration of glassware,
but to a lesser degree. An explanation could be
that they were simply more critical and percep-
tive of the trial glasses. However, it appears that
this spontaneous disintegration can occur in
new glassware and not just worn glassware, as
previously assumed. Bar workers need to be
made aware of the potential hazards and both
licensed premises workers and customers need
to be reassured that one pint capacity glassware
conforms to safety standards.
Despite experiencing problems with tough-
ened glass, the majority of staV still agreed that
the intervention glasses were “better” than the
glassware used previously. Many publicans and
their staV accepted that toughened glass is safer
and causes fewer injuries, but they were
presumably referring to intentional rather than
unintentional injuries. Because this study was
concerned with unintentional injury and be-
cause the “toughened” glassware was in fact
less impact resistant than annealed glassware,
this study does not contribute evidence of the
eVectiveness of toughened glass in reducing
assault injury. This study therefore highlights
the need to educate bar managers/landlords
about the diVerential risk in relation to
intentional and unintentional injury. Unfortu-
nately, many staV held unrealistic expectations
of toughened glass—that it doesn’t break when
dropped, or that it is completely harmless when
broken. This was well summarised in the words
of an experienced female publican: “although
the glasses [toughened] were stronger and easier to
Table 3 Impact resistance of one pint glasses
Glass type Batch size
Mean (SD) impact
resistance (J)
No intact after
1st attempt
No intact after
2nd attempt
Trial annealed 10 1.77 (0.15) 2 0
Trial toughened 10 1.38 (0.22) 3 2
Alternative toughened 10 N/A 10 9
N/A = not applicable.
Toughened glassware and injury in bars 39
collect, the staV tended to treat them with less care
and we tended to have more injuries”. Thus, it
appears that a proportion of injuries in the
intervention group may have been due to staV
complacency or carelessness.
In conclusion, this study demonstrates that
glass with lower impact resistance caused more
injuries, and that “toughened” glassware can
have lower impact resistance than annealed
glass. It thus emphasises the need for quality
control of the toughening process based on
published standards that need to be developed
as a matter of urgency.
Implications for prevention
x Increasing the impact resistance of bar
glassware reduces the risk of injury.
x The toughening process can increase
impact resistance substantially, without
altering the dimensions/thickness of glass-
ware, but it can also reduce glass impact
resistance.
x Quality control standards are urgently
needed to ensure that toughened glass
products are what they claim to be.
The authors are grateful to all the publicans and their staV who
participated in this trial, and to those who were willing, but not
included in the trial. For statistical advice, we acknowledge the
help of Dr Robert Newcombe, Department of Medical
Statistics, University of Wales College of Medicine, CardiV.For
testing the impact resistance, we thank Dr Mark Waters, Mate-
rials Laboratory, Dental School, University of Wales College of
Medicine. We also acknowledge the financial support of the PH
Trust.
1 Shepherd JP, Shapland M, Pearce NX, et al. Pattern, sever-
ity, and aetiology of injuries in victims of assault. JR.Soc
Med 1990;83:75–8.
2 Hocking MA. Assaults in South East London. J R. Soc Med
1989;82:281–4.
3 Shepherd JP, Price M, Shenfine P. Glass abuse and urban
licensed premises. J R. Soc Med 1990;83:276–7.
4 Shepherd JP, Huggett RH, Kidner G. Impact resistance of
bar glasses. J Trauma 1993;35:936–8.
5 Shepherd JP. Preventing injuries from bar glasses. BMJ
1994;308:932.
6 Evans DM. Hand injuries due to glass. J Hand Surg
1987;123:284.
7 Shepherd JP, Brickley MR, Gallagher D, et al. Risk of occu-
pational glass injury in bar staV. Injury 1994;25:219–20.
8 McLean W, Shepherd JP, Brann CR, et al. Risks associated
with occupational glass injury in bar staV with special con-
sideration of hepatitis B infection. Occup Med 1997;47:
147–50.
9 Shepherd JP. The circumstances and prevention of bar-glass
injury. Addiction 1998;93:5–7.
10 Nakamura H. Life of tumblers. In: Kunigi M, Tashio M,
Saga N, eds. Tenth international congress on glass.Tokyo:
Ceramic Society of Japan, 1974: 42–9.
11 Newcombe RG. Interval estimation for the diVerence
between independent proportions: comparison of eleven
methods. Stat Med 1998;17:873–90.
Overprotective parents?
Regional editor, Ian Scott, discovered a publication from the British Mental Health Founda-
tion in which it is argued that stress in children, such as bed wetting, are in part a reflection of
over-protectiveness. The author, Jane McKerrow, states, “children have become closeted by
obsessive parents who allow them little chance to take risks and learn from their own
mistakes”. Compounding this, in a related article in the British newspaper, The Independent,a
British sociologist (Frank Ferudi) places much of the blame on an over-zealous child protec-
tion industry. “British society behaves as though children were an endangered species. The
child safety industry uses every opportunity to promote its central message: that children are
permanently at risk from danger. Campaigns centred on a variety of subjects—child abuse, cot
death, roving paedophiles, sunbathing—all reinforce the belief that childhood is a particularly
dangerous experience”.
Editor’s comment: If the seemingly foolish premise were well supported scientifically I guess we
would have to plead guilty as charged with the mitigating argument that if the worst price we
must pay for keeping a child alive and free of major disability is bed wetting, so be it.
Aggressive children
A report in the Boston Globe cites a study by Field conducted at McDonald’s restaurant
playgrounds showing that French children acted aggressively 1% of the time while US children
did so 29% of the time (Dolores Kong, Boston Globe). It raises some fascinating questions about
how US boys are raised—for example, lack of touching in infancy, in relation to “the culture of
violence”. Field is concerned that there may be “even less physical aVection toward children in
US society as a result of teachers and daycare providers worrying about accusations of sexual
abuse. The newspaper report concludes with a quote from Harvard psychologist Dan Kindlon:
“We give boys an emotional miseducation. They grow up unable to talk about emotions or to
be aware of others’ feelings, and they have fewer ways of dealing with stress”.
40 Warburton, Shepherd
