|Year : 2014 | Volume
| Issue : 1 | Page : 50-55
Should area, population and sample selection for gas disaster studies be a multidisciplinary approach: Experiences from Bhopal MIC disaster
Brajendra Mishra, AN Seth
Department of Community Medicine, Chirayu Medical College and Hospital, Bhopal, Madhya Pradesh, India
|Date of Web Publication||25-Jun-2014|
Associate Professor, Department of Community Medicine, Chirayu Medical College and Hospital, Bhopal,462030
Source of Support: None, Conflict of Interest: None
Inroduction: Following methyl isocyanate (MIC) leak disaster in December 1984, epidemiological studies on health consequences among the affected population were started. The area, population and sample selection for these studies were mainly based on the mortality/morbidity experienced 72 hrs post-disaster or longer by the people living in affected area spread over in 36 wards. Designs for these studies were mainly cross-sectional or prospective. Very soon criticism to the method of area and sample selection started appearing in peer reviewed journals. Some scientists suggested alternative/improved methods for area/sample selection. In few studies, distance/exposure dose versus affect models were also developed and found to be more accurate. Authors decided to review the study design of Epidemiological studies conducted on Bhopal along with other gas disasters; two natural and two manmade have to occur during 1952-1986, for influence of non- epidemiological factors like terrain, meteorology contributed in precipitation of the disasters and the methodology used in area/population and sample selection. Materials and Methods: Field visits and Secondary data review: Authors visited Bhopal gas affected area many times in past and reviewed on other gas disasters. Results: Authors observed that area population and sample selection exercises can be strengthened by utilization of existing information on non-epidemiological factors contributing to disaster and information generated by other disciplines. It is concluded that in gas disaster epidemiological studies area, population and sample section should have multidisciplinary approach rather than being based on mortality morbidity indices or prevalence of effect rather than prevalence of the cause. Recommendations: Authors also recommend some steps to be considered before planning epidemiological studies on gas disasters in future.
Keywords: Atmospheric inversion, methyl isocyanate, patient location method, plume studies, temperature inversion, urinary thiocyanate levels
|How to cite this article:|
Mishra B, Seth A N. Should area, population and sample selection for gas disaster studies be a multidisciplinary approach: Experiences from Bhopal MIC disaster. Int J Health Syst Disaster Manage 2014;2:50-5
|How to cite this URL:|
Mishra B, Seth A N. Should area, population and sample selection for gas disaster studies be a multidisciplinary approach: Experiences from Bhopal MIC disaster. Int J Health Syst Disaster Manage [serial online] 2014 [cited 2018 Nov 17];2:50-5. Available from: http://www.ijhsdm.org/text.asp?2014/2/1/50/135371
| Introduction|| |
Major toxic gas leak disasters though occur infrequently but result in big catastrophe, killing a large number of people and affecting still larger number on long-term. Besides these similarities, these disasters have many things in common like sudden release of toxic gases and faster dispersal and disappearance in atmosphere within a very short period of time mostly in hours and rarely in days [Table 1]. Once these gases disappear, it becomes very difficult for epidemiologists or statisticians to find the evidence of their presence and dispersal. In such situations, mostly the mortality and morbidity rates or prevalence of these effects are taken as proxy for prevalence of cause as basic indices to identify the affected area and/or affected people. In other words epidemiologists and statisticians are left to use prevalence of the effects rather than prevalence and distribution or dispersion to be more precise for the cause of identification of afflicted area and selection of sample.
Bhopal Toxic Gas Disaster (2 nd /3 rd December 1984) was one such example. The longest running epidemiological study (1985-1994) based its area selection on mortality rates observed 72 hrs post-exposure among exposed/non-exposed population. The cohort for study was selected from the affected v/s control area based on the residential status in affected area and history of exposure/symptomatology following exposure, and besides general anthropometric, personal and socio-economic characteristics. The cohort was not evaluated for presence of toxic substances in body.  Later in a review of six studies on Bhopal methyl isocyanate (MIC) leak disaster, it was found that sampling methods were based on opportunistic, self-selected or volunteered cases.  Another review of 35 studies too confirmed that most of the studies were based on disease status rather than exposure.  Comparison of two maps developed on the basis of mortality [Figure 1] and on the basis of plume studies [Figure 2] at Bhopal following disaster conform to the point of view expressed by reviewers.
|Figure 1: Affected area on the basis of mortality (ICMR: Epidemiological study)|
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|Figure 2: Affected area of Bhopal on the basis of Plume study (Singh and Ghosh)|
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In present paper, Bhopal MIC disaster along with other four disaster occurred over the span of 34 years have been reviewed to understand the role of other factors in precipitating the disaster and the role of other specialties in defining the affected area as well as the affected population and also their role in preventing further catastrophe of similar nature. Attempts have also been made to short list the steps, which may help in designing the epidemiological studies on any future gas disaster, if ever occur.
| Materials and Methods|| |
The field visits: The area of epidemiological study (1985-1994) affected by Bhopal MIC gas leak was frequently visited by authors in capacity of co-investigator to supervise collection of mortality/morbidity data or as manager to supervise the health or delivery management.
Authors besides gathering first hand information form Bhopal gas disaster also reviewed other four major gas disasters caused by toxic gases since 1952 in chronological order to evaluate the role of other factors/discipline in defining the affected area as well as the affected population. These disasters include one natural, one natural precipitated by human activities and two man-made gas disasters.
| Results|| |
Since the disasters reviewed in this paper occurred over a period of 34 years (1952-1986), it becomes important to recapitulate the events, their settings and the outcome before observations are discussed.
The great smog of London: 1952
The ill famous London smog of 1952 lasting five days (5-9 th December 1952) led to very high mortality and morbidity. By first account it killed 4,000 people . A February 1953 report, raised this figure to 6,000 deaths and that of 25,000 person claimed of sickness benefits  Lastly in 2004, it is found that the fog accounted for 12,000 fatalities.  Most of the deaths were caused by respiratory infection/complications.  The smog was precipitated by an anti- cyclone settled over windless city leading to temperature inversion. And due to this phenomenon cold stagnant air remained trapped under the worm air.  Such cold conditions forced residents to heat their houses with burning coal and other polluting units like automobiles and factories continued spewing smoke unabated. The prevailing fog continued getting mixed in stagnant air for next five days daily with smoke particles (1000), carbon dioxide (200), hydrochloric acid (140) fluorine compounds (14) sulphur dioxide (370) and converted sulphuric acid (800). (All in tonnes). 
2, 4, 5 trichlorophenol leak disaster. 1976
At 12.37 pm of July 10 th , the Saturday 1976, a chemical plant named Industrie Chimiche Meda Societa Azionaria situated in Meda, Italy accidently released chemicals within a short span of 20 seconds. Approximately 3000 kg of released chemicals included 2, 4, 5 trichlorophenol, dioxin (100 gm to 20 kg). These chemicals went up to estimated 50 m into the sky,  and then carried southeast by the wind in form of a visible cloud.  The toxic cloud containing dioxi n contaminated a densely populated municipality of Seveso and other communities stretched over about 6 km long and 1 km wide area (18 km 2 ) lying downwind.  The disaster killed 3000 animals and about 200 people suffered with chloracne. The affected area was split into zones inhabited by 37,236 people on the basis of 2, 3, 7, 8-tetrachloro‐dibenzodioxin (TCDD) soil concentration.  In June 1977, a program for epidemiological health monitoring for 220,000 people was launched.
Bromine leak: Geneva 1984
On November 8, 1984 at 9.30 am 550 kg of liquid bromine leaked out from a plant at Geneva Switzerland. The dense brown cloud drifted north/north-eastward over the township along with the light wind till it reached the lake around 2.30 pm. The outside limit of the potentially contaminated zone was defined using Draeger tubes. The affected area was also identified by patient location method. More or less both areas matched. Bromine being heavier then air did not rise above the third storey of the building. The gas-affected population was estimated to be about 25,000. About 91 persons (0.4%) developed symptoms following gas exposure.Of the two methods used for identification of contaminated zones i.e 1- Draeger tube for presence of Bromine and 2. Patient location method for presence of effect. Patient location method seemed to work better as it detected additional 16 patients in lower area where due to limited technical staff and limited transport facilities investigating agency could not reach with Draeger tube. 
Bhopal toxic gas disaster: 1984
This disaster took place on 2 nd /3 rd December in a Sevin, a carbamet pesticide making Union Carbide factory situated in Bhopal. Accidentally some water entered into the tank and following the exothermic reaction, 27 tonnes of MIC and uncertain quantity of cyanide and other products of reaction in form of toxic gases leaked into atmosphere at 17°C, relative humidity about 60% and wind speed 10-11 km/hr flowing in North easterly  to north westerly  direction on the densely populated old city of Bhopal situated on comparatively planes  surrounded by high hillocks namely Idgah hills, Shymala hills and Birla hills on three sides.  The disaster, within 72 hrs killed about 2000 people.  Cross-sectional and prospective studies were planned on various affected organs/system of the affected people. Affected area was selected based on excessive mortality observed within 72 hrs post-disaster and the sample for epidemiological study too was drawn from the residents of the affected area (severe/moderate/mild). Later, it was observed that while developing affected area or sample for epidemiological studies, baring one or two trials, information from simultaneously running other studies [Table 2] on issues like gas dispersion, contamination and toxicology were not utilized in planning of epidemiological studies.
|Table 2: Bhopal toxic gas disaster: Information from multidisciplinary studies|
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CO 2 Leak Nyos Cameroon: 1986
In north western, Cameroon at 9.30 pm on 21 st August 1986 from otherwise placid and still lake Nyos a crater lake 1,800 m (5,900 ft) across and 208 m (682 ft) deep, formed some 400 years ago, 1.2 km 3 or 100,000-300,000 tonnes of CO 2 was suddenly released in 20 seconds at the speed of 100 km/hour to the height of 300 feet in air. Being 1.5 times heavier than air this 50 meter thick cloud of gas advanced down slope at the speed of 20-50 km/hr through northern lip of the lake in to valley. Later branching off to two, this cloud of gas killed about 1746 people, 3000 cattle and countless wild animals, birds and insects in worst affected villages of Cha, Nyos, and Subum within the distance of 23 km. Scientists also estimated that the lake Nyos as on 1987 possibly stored 1.45 km  of CO 2, and still has a potential of unleashing other disaster by releasing 0.38 km 3 of CO 2 as on 1987.  Scientists recommended degassing of CO2. 
| Discussion|| |
Seveso TCDD leak in 1976 and MIC leak disaster in 1984 have been water shed events in the history of industrial, especially in industrial gas disasters. Further, besides these disasters study of Nyos CO 2 leak, Geneva Bromine leak and London fog shed lights on the prominent role played by natural forces in causation of disasters. It is realized that information and experience gained form Bhopal and other sites need to be put in proper sequence, so that this acquired knowledge and experience help in planning of good epidemiological studies in case of any future gas disaster. In following pages, role of contributing factors operating backstage and influencing the occurrence, course and impact of disasters have been highlighted and how study of these factors can improve the planning epidemiological studies is discussed.
The landscape and terrain
The landscape of the affected area in Bhopal and London were flat while in case of Seveso, Geneva and Nyos the point of discharge of gases/chemicals were situated comparatively, uphill and the toxic gases/chemicals were either seen or recorded as drifting downhill in Nyos up to 23 km and in Seveso up to 6 km. It is noted that in Geneva bromine being heavier than air did not rise above the three-storey building.
The meteorological conditions
In London meteorological condition like natural phenomena of anticyclone, temperature inversion and windless atmosphere became conducive to the accumulation of the toxicants, which otherwise would have blown away. Similarly, in case of Bhopal meteorological conditions played major role in causation of the disaster. The low night temperature of 17°C resulted in phenomena of atmospheric inversion and low wind speed pushed the condensed toxic cloud towards the city. Role of meteorological conditions on impact of disaster can be visualized by one hypothetical scenario. Imagine Bhopal MIC leak to occur at 3.00 pm on 6 th June having outside temperature of about 40°C. In such scenario MIC (BP 39°C) and hydrogen cyanide (BP 26°C), both constituents of the released product of reactions would have probably blown away to high altitude without killing a single person or even without being noticed by the lay people around. Similarly, in case of Bhopal, Geneva and Seveso toxic gases were seen blowing with the wind on plain area or downhill. And lastly, in London water droplets of the fog contained the smoke and windless atmosphere led to stagnation of the same over the residential quarters.
Identification of area of affliction
Since the epidemiology has developed from infectious diseases a general concept of equi-directional, equi-dispersion of the cause in the area of affliction has been carried through. However, in practice, it is not seen universally especially in situations when a vehicle is involved. The study of cholera by John Snow is one such example. People developed the diseases that drank infected water. The second example, which conforms to this observation is that of Minimata disease. All those who consumed contaminated fish developed the diseases.  Before this argument is taken further two maps used in describing the affected area in Bhopal need a look. [Figure 1] or Map A has been used in one longest running epidemiological study. In this map severely affected area is surrounded on three side by moderately affected area and then by the mildly affected area in centrifugal manner. And, this distribution is very close to the concept of uniform equi-directional, equi-dispersion of the cause. Now let us have a look at the [Figure 2] or Map B. This map was drawn on the basis of plume studies considering the meteorological conditions and the wind direction. Map B contrary to the Map A depicts the affected area in unidirectional way starting from the severe area close to factory in north easterly direction than flowed towards very badly, moderately and lastly mildly affected area in south westerly direction. And that is how the direction of wind flow was reported to have occurred on the night of 2 nd /3 rd December 1984. Map B also matches with the mortality rates as well. Other major difference between Map A and B is that in Map A mild area is spread north to the railway line (RL), whereas in Map B area north to the RL is shown as unaffected as the toxic gases did not traversed to that direction/area.
There are many modalities that can help in identification of the area of affliction. Firstly, visualization of plume as in Geneva, Bhopal and Seveso can indicate the gross area of affliction. Visualization of damage on stationary or immovable biosystems like defoliation on plants and death of harnessed animals as in Bhopal Nyos and Seveso can again help in identification of the area of affliction. Plume modeling as done in Bhopal with concentration gradient can help in identification of area of affliction more scientifically. Here, it is worth mentioning that though plume modeling with concentration gradient was attempted in Bhopal but the information was not used for epidemiological studies. Monitoring of atmospheric bromine concentration was used to define the affected area geographically.  In Geneva, patient localization method was used to define the affected area and proved more effective than the use of Draeger tubes for detection of bromine. On ground, in Seveso, concentration gradient of the toxicant (TCDD) deposits in soil helped in identification of affected area whereas in Bhopal though possibility of presence of methylamine a product of reaction in soil and water was identified,  but no attempt was made to develop concentration gradient and use it in identification of the area of affliction.
Identification of the affected population
After identification of area of contamination and its zoning in to different gradient of affliction, it becomes important to identify the people, who were affected by gas leak. In Bhopal, patients were categorized to conform seriously, moderately or mildly affected area. But, this does not mean that person from seriously affected area was also clinically and seriously affected. And, this was the limitation of registry approach. To overcome this, a model on distance/exposure dose affect was suggested.  Cohort approach vis a vis registry approach was a further refinement in this direction.  It goes without saying that cohort would include exposure related data including toxicant levels in blood as well. Here, it is worth mentioning that in Bhopal over 18 months of post-disaster, about 18,000 urine samples were monitored for urinary thiocyanate levels.  Though looking into the size of affected population of 500,000 and above, this number seems less but larger number of urine samples could certainly have helped in better identification of the affected area as well as in development of the cohort for epidemiological study. Koplan in same reference opines that epidemiologic studies following disasters should accurately estimate exposure to enable correct dose response relationship modeling, as it could have helped in solving issues like (a) identifying exposed and ill persons, (b) determining long-term effects and (c) linking exposure and effects for use in litigation and to determine compensation. 
Lastly, job of epidemiology cannot be considered as done unless attempts are made to prevent future recurrence of the disaster. By estimating the quantum of the remaining toxicants in the facility or the source of the disaster like at Nyos and Bhopal the Geologists/toxicologists contributed to the amelioration of further disaster.
In present paper, authors have tried to emphasize two issues: one is the role of the terrain and meteorological conditions in precipitation of the gas disasters and second role of other disciplines in decision making for selection of area and the sample for epidemiological studies. As submitted earlier in this paper that while planning of epidemiological studies, knowing or unknowingly two components are considered a) the prevalence of the cause and b) the prevalence of the effect. At several instances, prevalence of the effect is taken as proxy for prevalence of cause as happened in Bhopal.
Hence, on the basis of the experience gained from the epidemiological studies at Bhopal and the review of literature on other disasters, authors suggest that following points should also be considered before planning the epidemiological studies on gas disaster:
- Chemical nature of the gas/toxicants their reactivity with environment, soil water. Persistence and level of the product of reaction in environment
- Meteorological conditions
- Plume studies
- Extent of dispersion based on plume studies and the presence of product of reaction in air, water and soil, if needed
- Tell-tell evidence of disaster on plants and animals specially harnessed one
- Location of patients when they developed first symptoms or noticed disaster
- Develop map of the affected area based on above information
- Blood levels of toxicants and their concentration gradient in reference to their distance from the source of the disaster at the time of developing first symptom
- Develop cohort of the affected population on the basis of the above and the cohort should also include the toxicant levels in blood to show the dose-response relationship while considering delayed effects later.
The list is not complete, exhaustive or all inclusive and may differ from case to case, but keeping these points in mind may come handy at the time of planning epidemiological studies on gas disaster.
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[Figure 1], [Figure 2]
[Table 1], [Table 2]