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Email “ sreeraj.nair[at]
Industries handling dangerous materials continuously
identify hazards and manage risk. One of the major
hazards in process or manufacturing industry is the
release of toxic chemicals. The hazard arising from a
toxic release depends on the toxicity of the chemical
and the conditions of exposure. The effects from the
exposure to toxic release could be acute or chronic.
Acute effects result from a single exposure to a high
concentration of the chemical whereas chronic effects
result from exposure to low concentration, but for a
longer duration.
The hazard from a release of a toxic substance
could be disastrous, and hence is classified as a
major hazard along with major fires and
explosions. The effect from exposure to a toxic
substance could be immediate or gradual and this
is determined by the entry routes and the toxicity
Potential toxic hazards can be assessed for a
number of purposes from land use planning to
emergency response.
determines toxic effects and considers the effects
based on the criteria set for each purpose.
Toxic assessment criteria are set to suit the
regulatory framework, national/regional standards
and the purpose of assessment. However, many
authorities and regulators believe that it is
important in regulatory toxicology to use
consistent and transparent methodology.
Key words Toxic hazard, toxicity, consequence
assessment, toxic dose, dangerous toxic load,
SLOD, SLOT, toxic criteria, major accident hazard
During early years of industrial operation,
hazards from toxic chemicals were perceived
primarily in terms of chronic exposure and were
not well appreciated. Over a period of time and
through developments in the field of chemical
engineering, the threat of large scale acute
effects were acknowledged. It took some major
incidents for the industry to look seriously at
toxicity issues.
Toxic release incidents
The toxic hazard effect range from a toxic hazard
can be far-reaching; often the release of a very
toxic chemical under unfavourable conditions is
considered to have a disaster potential greater
than that of a fire or explosion. Unlike the fire
and explosion incidents, a major toxic release
does not have a major impact on the installation
(or the plant facilities), but the effect on
environment and public population could be
significant and take longer to recover from.
Some of the major incidents involving significant
toxic release and the severity are given below
[Mannan (2005), Kletz (2001)]:
x Bhopal gas tragedy, India (3rd Dec 1984),
highly toxic methyl iso-cyanide release resulted in
about 2000 fatalities and tens of thousands
x Montana, Mexico (1st August 1981), incident
involving chlorine rail tank car resulted in 17
death and 280 injuries.
x Seveso disaster, Italy (10 July 1976), a
discharge containing highly
contaminated a neighbouring village over a
period of 20 minutes. About 250 people
developed chloracne (skin disease), about 450Page 11

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 2
were burned by caustic soda. A large area of land
17 km
was contaminated and about 4km
made uninhabitable. About 80,000 animals died
or were killed to prevent contamination filtering up
the food chain.
x Zarnesti, Romania (1939), failure of chlorine
storage tank resulted in the death of about 60
Toxic Hazard Management
As the industry and the authorities became more
aware of the hazards, guidance was developed
and regulations enforced to identify and manage
the toxic hazards.
In order to manage the risk from toxic hazard, the
consequences from potential events need to be
understood, assessed and analysed against
suitable criteria. However, defining toxic criteria is
complex process. This is due to:
x the number of routes of exposure (inhalation,
ingestion and external contact)
x the length of exposure
x the frequency of exposure
x physiological effects and individual response
to the exposure.
Defining Toxic Criteria
Toxic criteria are normally set based on the
applications or purpose and in general can be
categorised as:
x Hygiene standards
x Emergency exposure limits
x Major accident hazard and land use planning
How each of these types of toxic criteria can be
used is described.
Hygiene standards
There are two principal sets of occupational
hygiene standards in use:
x Threshold Limit Values (TLV)
x Occupational Exposure Limits (OELs)
TLV is expressed either in terms of parts per
million (ppm) or mg/m
and is intended for use
in industrial hygiene applications only. The three
categories of TLV are:
(1) Threshold limit value, time-weighted average
(TLV-TWA “ concentration for a normal 8 hr
work day or 40 hr work week to which nearly all
workers may be repeatedly exposed, day after
day, without adverse effect.
(2) Threshold limit value short-term exposure
limit (TLV-STEL) - maximum concentration to
which workers can be exposed for a period of
15 min continuously without suffering from
intolerable irritation, chronic or irreversible
tissue change.
(3) Threshold limit value-ceiling (TLV-C).-
concentration that should not be exceeded even
An international set of Occupational Exposure
Limits (OELs) for Airborne Toxic Substances, is
published by the International Labour office
(ILO, 1991/2).
In addition to these principal sets there are
various other hygiene standards in use. For
example, the limits set by Occupational Safety
and Health Administration (OSHA) include:
x Maximum acceptable
regardless of period of exposure.
x Permissible exposure limit (PEL): applied
variously to the TLV, STEL or MAC.
x Action level (AL): one-half the PEL; the
which additional
measurements of the same exposure will
probably not exceed the PEL.
Another criteria referred to in the Control of
Substances Hazardous to Health (COSHH)
regulations is Occupational Exposure Standard
(OES), a concentration at which there is no
indication of risk to health. OESs are listed in
EH40/2005 by HSE (2007). Effects from some
substances could be sensitizer effects and
asphyxiant.Page 12

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 3
which influence hygiene
standards relate to health effects such as
substances which are carcinogenic, dusts or
Emergency exposure limits
Some emergency exposure limits, emanating
from various bodies, include the following:
x Emergency exposure guidance level (EEGL)
x Emergency exposure index (EEI)
x Emergency exposure limit (EEL)
x Emergency exposure guidance level (EEGL)
x Emergency exposure index (EEI)
x Emergency exposure limit (EEL)
x Emergency response planning guideline
x Immediately dangerous to life and health
(limit) (IDLH)
x Public emergency exposure limit (PEEL)
x Short-term public emergency guidance level
x Indicative occupational exposure limit valves
These emergency exposure limits are all
intended to be used to support emergency
planning and response. For example, the ERPG
is the maximum airborne concentration below
which, it is believed; nearly all individuals could
be exposed for up to 1 hr without experiencing or
developing certain defined effects. Three ERPGs
are used, the defined effects being as follows:
x ERPG-1 Effects other than mild transient
adverse health effects or perception of a
clearly defined objectionable odour.
x ERPG-2 Irreversible or other serious health
effects or symptoms that could impair an
individualâ„¢s ability to take protective action.
x ERPG-3 Life threatening health effects.
There are also indices for acute toxic exposures
that are not based on a simple concentration
value but take account of other parameters.
An example is the Chemical Exposure Index
(CEI) developed by the Dow Chemical Company
[Mannan (2005)], which takes account of toxicity,
quantity, distance, molecular weight and process
variables. The index is computed as the product
of a set of scale factors.
Major Accident Hazard and Land Use Planning
The third area where toxic criteria are used is in
risk assessment for major accident hazard and
land use planning. For this use an adequate and
appropriate vulnerability model for exposure to a
toxic chemical is required in order to predict the
degree of harm from a given level of exposure.
The harm criteria can be used to estimate the
consequence from the exposure that, when
combined with the likelihood of the exposure
equates to the risk from the hazardous event.
Examples of the different methods of estimating
the vulnerability from a potential major hazard
x Probit Equations
x Toxic Index
x Green Book Relations
x Dangerous Toxic Load
Probit Equations “ Toxicity
Probit equations were developed during 70s and
and were
tentative and
approximated. Probit equations were used in the
Canvey Report and Rijnmond Report [Mannan
(2005)] and have been widely used in hazard
assessment since then.
Probit equations are available for a number of the
toxic gases of industrial interest. They include in
the collections
given for
Vulnerability model by Perry and Articola (1980),
in the QRA Guidelines by the CCPS (2000) and in
the Green Book by the CPD (1992).
The harm (Y) is derived from Y = a + b Ln(V)
Where, V is the variable quantifying the physical
cause of injury;
a and b are constants corresponding to the toxic
chemical.Page 13

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 4
The probit value derived is converted to an
estimate of the percentage of fatalities in the
exposed population using standard probit tables
[Mannan (2005)].
The probit equation for chlorine lethality derived
by Eisenberg et. al (1975) is
Y = -17.1 + 1.69 ln (?C
Where, Y is the probit, C is the concentration
(ppm), T is the time interval (min), and n is an
index. From analysis of the data, they obtained a
value of n = 2.75.
The revised version of the Eisenberg equations
by Perry and Articola (1980) is
Y = -36.45 + 3.13 ln(?C
Further, the industrial comment on the Rijnmond
Report (Rijnmond Public Authority, 1982)
proposes the equation:
Y = -11.4 + 0.82 ln (?C
Toxic index
The toxic index method is an assessment based
on a combination of the worst case release,
consequent generation of vapour and a limiting
tolerable concentration which is judged not to
cause irreversible effects.
Weighting factors based on material, process
and layout features of the unit are assigned and
the final index value is derived using a simple
formula. A seven point ranking scale is provided,
based on comparison with a selection of actual
units. There is a close analogy with the ranking
of fire and explosion hazards by the Mond Index,
and this toxicity index can be used alongside the
Mond Index. More details are given in Mannan
Green Book Relations
Another means of assessing the toxicity of
industrial gases is that given in Methods for the
Determination of Possible Damage by the
Committee of Prevention of Disasters in the
Netherlands (1992) (the Green Book) based on
research by The Netherland Organisation. An
account is given by deWeger et. al (1991).
In this methodology a distinction is made
systemically acting substances.
Dangerous Toxic Load
The Dangerous Toxic Load (DTL) describes the
exposure conditions, in terms of airborne
concentration and duration of exposure, which
would produce a particular level of toxicity in the
general population. One level of toxicity used by
United Kingdom (UK) Health and Safety
Executive (HSE) in relation to the provision of
land use planning (LUP) advice is termed the
Specified Level of Toxicity (SLOT).
Hazardous Installations Directorate (2008) has
defined the LUP SLOT as:
Severe distress to almost everyone in the area
Substantial fraction of exposed population
requiring medical attention
Some people seriously injured, requiring
prolonged treatment
Highly susceptible people possibly being
The basis of the toxicology assessment
The toxicity of a given substance in the air is
influenced by two factors, the concentration in the
air © and the duration of exposure (t). A
functional relationship between c and t can be
developed, such that the end product of this
relationship is a constant:
f(c,t) = constant
This constant is known as the Toxic Load. In
HSE, the Toxic Load relating to the LUP SLOT is
known as the SLOT Dangerous Toxic Load or
SLOT DTL. For a number of gases the
relationship between c and t is simple:
Toxic Load = c x t
This relationship is sometimes known as the
Haber law. As an example, animal toxicity data
For methyl isocyanate indicates that the LUP
SLOT is produced by each of these c and t pairs:Page 14

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 5
t (minute)
c (ppm)
In this example the constant, or SLOT DTL, is
750 ppm.min (that is 150 x 5, 25 x 30, etc.).
However, the equation c x t = constant does not
apply to all substances, so the following general
equation has been developed:
Toxic Load = c
For methyl isocyanate, n in the c
.t relationship
is 1. In the case of chlorine, n = 2 and animal
toxicity data suggest that the following pairs of c
and t will each produce the SLOT:
t (minute)
c (ppm)
Here, the constant, or SLOT DTL, is 1.08 x 10
For chlorine, hence the concentration versus
time relationship is not linear (n=2), implying
exposures to very high concentrations for a few
minutes may produce more damaging effects
than lower concentrations over a longer period.
Determination of the DTL
The information concerning accidental chemical
exposures to humans causing severe toxicity is
derived from exposure mortality data (usually
tests over a known duration) designed to
identify exposure conditions that produce
mortality in 50% of a group of animals.
The SLOT criteria reflect the exposure
conditions just on the verge of causing a low
percentage of deaths in the exposed population.
Hence, conditions producing around 1%
mortality (LC
) in animals are taken as being
representative of SLOT conditions. In deriving
the DTL, the available acute toxicity data from
different species is compared and the data from
the most sensitive animal species is used.
A similar procedure is followed to derive a toxic
load equation to predict exposure conditions
producing any other specified level of toxicity
that may be of interest. For example a DTL
relating to the mortality of 50% of an exposed
population, a specified level known as the
Significant Level of Death (SLOD) DTL, can be
determined (see Mannan et al (2005) for more
There are many limitations to the approach
extrapolating animal data to humans, lack of
relevant toxicity data, the use of animal data of
poor or unknown quality, frequent use of the
default assumption that n in the c
t = DTL
equation is equal to 1 and uncertainties about
the universal applicability of the c
t concept.
However, the described approach is probably
the best that can be achieved with the available
data and current state of scientific research.
The use of toxicology data in risk
assessments and LUP
When assessing toxic risk, assessors are
required to estimate the extent (i.e. hazard
ranges and widths) and severity (i.e. how many
people are affected, including the numbers of
fatalities) of the consequences of each identified
major accident hazard.
For an evenly distributed population, the number
of fatalities resulting from a toxic release may be
approximated by estimating the number of
people inside the concentration contour leading
to an LD
dose (i.e. SLOD DTL).
Further, the number of people injured (serious
and minor) by the release may be approximated
by the number people estimated to be between
the SLOD and SLOT DTL contours (i.e. the Page 15

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 6
SLOT DTL contour is taken as a pragmatic limit for
Decisions of the acceptability of a new hazardous
considering the
surrounding populations, or the implication of the
existing hazard zones from an (industrial or
domestic) to be located at a certain distance from
the installation, can be developed from the risk
estimated based on the severity of the potential
major hazards.
The use of DTL in LUP is illustrated in the
following example
This example assesses an application for planning
permission for a new industrial installation which
stores and handles chlorine.
A consequence
assessment with DTL criteria is carried out to
determine the toxic hazard range to define the
hazard zone.
Scenario: Catastrophic rupture of 40000kg
chlorine storage vessel.
Consequence modelling tool: DNV PHAST v6.53.1
Location of the installation: South Kalamassery,
Kochi, India
The input data for the consequence modelling for
scenario is as given in the Table 1.
Description: This case models an instantaneous
release of the entire vessel inventory. The release
is assumed to form a homogeneous mass,
expanding rapidly in all directions. In this example
the release is located close to the ground, so
forms a hemispherical cloud. After initial expansion
it moves in one direction downwind until it no
longer contains harmful concentrations.
This assessment has estimated the hazard ranges
relating to the probability of exceeding the SLOT
and SLOD dose. The following assumptions shall
be made for the calculation of fatalities and injuries
for outdoor populations:
x 100% fatalities between the release point and
the SLOD contour
x 10% major, 90% minor injuries between the
SLOD and SLOT contours
The DTL SLOD and SLOT contours plotted
using PHAST is given in the Figure 1.
Discussion: The SLOD (green) contour
extends to about 2 km and the SLOT contour
extends to 3 km down from the release source.
In the event of such a release near a densely
populated area there would be a considerable
number of fatalities and injuries.
In this
example case, the event could result in
hundreds and thousands dead of injured if the
installation is allowed near a city or town like
The results of this kind help those with the
responsibility for the granting of planning
permission to make decisions regarding a
proposed new installation at a given location or
conversely for a domestic development or a
public building (hospital or school) near an
existing high hazard installation.
Disclaimer: This is an example only and is not
intended to reflect any intention to site a
chlorine facility at this location.
Over a period of years and after many major
incidents, the industry has realised the
potential hazard from of handling toxic
substances. Globally, many standards and
codes are available for different purpose
ranging from occupational hygiene to land use
Industry has learned a lot from previous
incidents and implemented a lot of risk
reduction measures. However, incidents are
still happening and many people are killed or
injured. It is an accepted fact that there is a
certain risk
operating hazardous
installations and a continuous effort needs to
be made to maintain the risk at as low a level
as practicable.
In line with the quote by the process safety
guru Tevor Kletz what you don't have can't
leak, it is also true that those who are not
there canâ„¢t be affected.Page 16

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 7
Table 1 PHAST Input Data
Weather: F 1.5m/s
Study Folder: Chlorine release_Kochi example
Model: Chlorine Rupture
Material: CHLORINE
Height: 0 m
Averaging Time: Toxic(600 s)
Audit No: 7269
Outdoor Toxic
Figure 1 Chlorine Release “ DTL for LUP
DTL SLOT (108 000 ppm
.min) contour
DTL SLOD (484 000 ppm
.min) contourPage 17

Journal of HSE & Fire Engineering
Issue 2 March 2009
Page 8
In order to keep the risk from toxic hazard low, it is
important where possible to exclude populations
from the vulnerable zone. It should be noted that
the key lies in determining and using the toxic
assessment criteria appropriately.
1. Center for Chemical Process Safety (2000)
Evaluating Process Safety in the Chemical
Industry: A User's Guide to Quantitative Risk
Analysis, AIChE, New York, US.
Committee for the Prevention of Disasters
(1992). Methods for the Determination of
Possible Damage to People and Objects
Resulting from Releases of Hazardous
Materials. Rep. CPR 16E (Voorburg) (the
ËœGreen Bookâ„¢

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