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.

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)]:

·       Bhopal gas tragedy, India (3rd Dec 1984), highly toxic methyl iso-cyanide release resulted in about 2000 fatalities and tens of thousands injured. 

·       Montana, Mexico (1st August 1981), incident involving chlorine rail tank car resulted in 17 death and 280 injuries.

·       Seveso disaster, Italy (10 July 1976), a discharge containing highly toxic dioxin contaminated a neighbouring village over a period of 20 minutes. About 250 people developed chloracne (skin disease), about 450 were burned by caustic soda. A large area of land 17 km² was contaminated and about 4km² was made uninhabitable. About 80,000 animals died or were killed to prevent contamination filtering up the food chain.

·       Zarnesti, Romania (1939), failure of chlorine storage tank resulted in the death of about 60 people.

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:

·       the number of routes of exposure (inhalation, ingestion and external contact)

·       the length of exposure

·       the frequency of exposure

·       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:

·       Hygiene standards

·       Emergency exposure limits

·       Major accident hazard and land use planning limits.

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:

·       Threshold Limit Values (TLV)

·       Occupational Exposure Limits (OELs)

TLV is expressed either in terms of parts per million (ppm) or mg/m³ 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 instantaneously.

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: 

·       Maximum acceptable ceiling (MAC): maximum exposure concentration regardless of period of exposure.

·       Permissible exposure limit (PEL): applied variously to the TLV, STEL or MAC.

·       Action level (AL): one-half the PEL; the concentration below 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.

Other hazards which influence hygiene standards relate to health effects such as substances which are carcinogenic, dusts or metals.

Emergency exposure limits

Some emergency exposure limits, emanating from various bodies, include the following:

·       Emergency exposure guidance level (EEGL)

·       Emergency exposure index (EEI)

·       Emergency exposure limit (EEL)

·       Emergency response planning guideline (ERPG)

·       Immediately dangerous to life and health (limit) (IDLH)

·       Public emergency exposure limit (PEEL)

·       Short-term public emergency guidance level (SPEGL)

·       Indicative occupational exposure limit valves (IOELVs)

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:

·       ERPG-1 Effects other than mild transient adverse health effects or perception of a clearly defined objectionable odour.

·       ERPG-2 Irreversible or other serious health effects or symptoms that could impair an individual’s ability to take protective action.

·       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 are:

·       Probit Equations

·       Toxic Index

·       Green Book Relations

·       Dangerous Toxic Load

Probit Equations - Toxicity

Probit equations were developed during 70s and 80s and were avowedly 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 particular the collections given for the 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.

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 (SC _i ⁿ T_i)

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(SC _i^2.64T)

Further, the industrial comment on the Rijnmond Report (Rijnmond Public Authority, 1982) proposes the equation: Y = -11.4 + 0.82 ln (SC _i2.75 T_i)

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 (2005).

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 between locally acting substances and 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 every one in the area

·         Substantial fraction of exposed population requiring medical attention

·         Some people seriously injured, requiring prolonged treatment

·         Highly susceptible people possibly being killed.

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 (c) 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:

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 = cⁿ.t

For methyl isocyanate, n in the cⁿ.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:

Here, the constant, or SLOT DTL, is 1.08 x 10⁵ ppm².min.

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 LC₅₀ 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 (LC₁) 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 information).

There are many limitations to the approach described above, such as difficulties 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 cⁿt = DTL equation is equal to 1 and uncertainties about the universal applicability of the cⁿt 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 LD₅₀ 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 SLOT DTL contour is taken as a pragmatic limit for injuries).

Decisions of the acceptability of a new hazardous installation (or modification) considering the surrounding populations, or the implication of the existing hazard zones from an installation on proposed new developments (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.

Table 1 PHAST Input Data

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:

·       100% fatalities between the release point and the SLOD contour

·       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.

Wind direction

North

DTL SLOT (108 000 ppm2.min) contour DTL SLOD (484 000 ppm2.min) contour

Figure 1 Chlorine Release – DTL for LUP

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 Kochi.

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

Conclusion

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 planning. 

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 from 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 can’t be affected”. 

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.

References

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

2.      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 ‘Green Book’)

3.      Eisenberg, N.A.., Lynch, C.J. and Breeding, R.J. (1975). Vulnerability Model: A simulation system for assessing damage resulting from marine spills. Rep. CG-D-136_75. Enviro Control Inc., Rockville, MD

4.      Hazardous Installations Directorate (2008), Assessment of the Dangerous Toxic (DTL) for Specified Level of Toxicity (SLOT) and Significant Likelihood of Death (SLOD); Health and Safety Executive, UK http://www.hse.gov.uk/hid/haztox.htm

5.      Health and Safety Executive (2007), Workplace Exposure Limits: Containing the list of workplace exposure limits for use with the Control of Substances Hazardous to Health Regulations 2002 (as amended), Series No. EH40/2005, HSE Books, UK

6.      Kletz, T.; Learning from Accidents, 3rd edition, Elsevier, Oxford, UK and St Louis, MO, 2001.

7.      Mannan, S. et. al (2005), Lee’s loss prevention in the process industries: hazard identification, assessment and control, 3rd Edition, Elsevier Butterworth – Heinemann, UK

8.      Perry, W.W. and Articola, W.P. (1980). Study to Modify the Vulnerability Model of the Risk Management System. Rep.CG-D-22-80. Enviro Control Inc., Rockville, MD

9.      Rijnmond Public Authority (1982). Risk Analysis of Six Potentially Hazardous Industrial Objects in the Rijnmond Area, A Pilot Study (Reidel: Dordrecht)

10.  deWeger, D., Pietersen, C.M. and Reuzel, P.G.J. (1991). Consequences of exposure to toxic gases following industrial disasters. J. Loss Prev. Process Ind., 4, 272