The impact of catalytic converters on motor vehicle exhaust gas suicides

Abstract

Introduction

In 1995, suicide by motor vehicle exhaust gas accounted for 509 deaths and was second only to hanging. It was most commonly used by males (85%), especially those aged 20-50 years. Rates of motor vehicle exhaust gas suicide per 100 000 population in Australia were 4.69 for males, 1.02 for females and 2.85 total (Australian Institute for Suicide Research and Prevention, 1996; unpublished data). Further, motor vehicle exhaust gas related self-harm represented a smaller proportion of patients hospitalised for attempted suicide than of successful suicides (2.0% v. 21.6%; Australian Institute of Health and Welfare, National Injury Surveillance Unit, 1996; unpublished data), indicating the high lethality of this means of suicide.

Motor vehicle availability has been found to be related to exhaust gas suicides,¹ and in Australia the rates of such suicides in different States vary to some extent with motor vehicle registration rates. For example, the Northern Territory has the lowest rates for each, while Western Australia has relatively high rates (Australian Institute of Health and Welfare, National Injury Surveillance Unit, 1996; unpublished data).²

Carbon monoxide (CO), the most toxic component of motor vehicle exhaust gas, is colourless, odourless and tasteless and is produced from the incomplete combustion of organic fuels. Mild CO poisoning produces headache and drowsiness; more severe poisoning leads to collapse, coma and death.³ Other effects can include myocardial infarction, deterioration of personality and impaired memory.^4,5

Environmental regulations have required carbon monoxide emission levels from vehicle exhausts to be reduced over past decades. The required maximum levels have been 24.3 g/km from July 1976 (Australian Design Rule [ADR] 27A), 9.3 g/km from 1986 for new passenger vehicles (ADR 37-00) and 2.1 g/km for new models (ADR 37-01) from 1997 and for all new passenger vehicles from 1998. To cope with the unleaded petrol legally required since 1986 to reduce environmental air pollution from exhaust emissions, vehicles usually require catalytic converters (ADR 37-00; Australian Standard 2877). These convert carbon monoxide and other pollutants into the by- products, carbon dioxide and water.

The purposes of this study were to examine overall trends in exhaust gas suicides, and to investigate the impact of the 1986 reduction in allowable carbon monoxide emissions (or the effects of catalytic converters) on that suicide rate.  

Methods

Data on vehicle numbers, rates per head of population and year of manufacture were obtained from ABS publications. Australian exhaust gas suicides and motor vehicle registration rates were compared for the ABS Motor Vehicle Census years since 1970.

To examine the effects of catalytic converters, we undertook a retrospective stratified case series study of 100 exhaust gas suicides in Victoria between 1 January 1994 and 31 December 1996, noting the year of manufacture of the motor vehicles involved and victims' blood levels of carboxyhaemoglobin (COHb) -- a reflection of carbon monoxide levels. We compared COHb levels in victims whose suicides involved vehicles manufactured before 1986 with those whose vehicles were manufactured from 1986 (when CO emission levels were reduced). Cases of exhaust gas suicide in 1994 were obtained from a Victorian State Coroner's Office publication,⁶ while those in 1995 and 1996 were obtained from the Victorian Coroner's Facilitation System database.  

Results

Hospital admissions for Australian motor vehicle exhaust gas suicide attempts have increased steadily in recent years (1994/95 data are not available), with the data showing an exponential trend (R² = 0.99). Deaths also increased, but show no clear trend (Figure 2).

The 100 cases of exhaust gas suicide obtained from the records of the Victorian Coroner comprised 33 of a total of 96 in 1994, 33 of 140 in 1995, and 34 of 136 in 1996. The year of vehicle manufacture had been recorded for only 75 of these cases (25 in each of 1994, 1995 and 1996). Of these 75, 20% in 1994, 56% in 1995 and 32% in 1996 involved vehicles manufactured in 1986 or later. In total, 27 of the 75 vehicles (36%) were 1986 models or later. This proportion is not significantly different (P = 0.62) from the 39% of vehicles manufactured in 1986 or later in the total Victorian fleet of 2 799 310 vehicles in May 1995 (ABS 1995 Motor Vehicle Census).²

For the 74 cases in which both the victim's COHb level and the age of the vehicle was known, 26 involved vehicles manufactured in 1986 or later and 48 involved earlier models. The median COHb level of victims in these two groups was identical (77.1%).  

Discussion

In 1995, 43% of registered Australian motor vehicles were manufactured in 1986 or later.² If exhaust gas suicide had been made impossible by catalytic converters and the 1986 reduced CO emission standard of 9.3 g/km, then rates for such suicides should have reduced from 2.08 to about 1.19 per 100 000 population between 1985 and 1995 (assuming a similar distribution of vehicles to that of the Australian fleet and an unchanged number of attempts). In fact, rates per 100 000 population have been about 2.5 for the past five years and peaked at 2.85 in 1995.

It is clear from our investigation of the sample from the Victorian Coroner's files that vehicles manufactured since 1986 have been used in exhaust gas suicides. Our finding that there was no difference in median COHb levels between victims who used vehicles with and without catalytic converters is consistent with the results of a New South Wales study.⁷ Given that CO emissions from vehicles with catalytic converters are lower, it can be deduced that time (and possibly other factors) need further investigation.

The few overseas studies which have examined the relationship between suicides and motor vehicle exhaust gas have found that imposing emission controls reduces the incidence of such suicides.^8-10 Vehicle emission limits for CO in the US have been lower than those in Australia, being at our 1986 limit (9.3 g/km) as early as 1975 and at just under our 1997 limit (2.1 g/km) in 1981.¹¹ Thus, US trends should shed some light on the Australian situation, especially as the test conditions are similar. However, it should be noted that the average age of vehicles in the US is seven years, compared with 11 years in Australia, which suggests that there would be a longer lag time for exhaust emission changes to affect suicide rates in Australia.

Motor vehicle exhaust gas is a less popular method of suicide in the United States than in Australia (1991 rates per 100 000 population being 0.73, compared with 2.78 for Australia).¹² Data available for the US to 1991 show a decline in the exhaust gas suicide rate from the years 1963 to 1979, an increase from 1981 to 1987, followed by a decline to below the pre-1982 rates. Exhaust gas suicides in the US have decreased from 8.9% of suicides in 1970 to 7.4% in 1980 and 5.6% in 1991.^8,12

Despite some conflicting evidence in Australia and the US as to the reductions in CO exhaust emission levels ne cessary for influencing exhaust gas suicide rates, these emission reductions do not appear to have produced the success suggested by early reports in the international literature.^8-10 There are a variety of possible explanations for this. Firstly, exhaust gas suicides usually involve a hose or pipe being used to feed exhaust gas from an idling vehicle into the sealed interior of a vehicle. Clearly, CO emission levels set for environmental reasons may not be particularly relevant in this situation, and if suicides are to be prevented it would be more appropriate to set CO emission limits in relation to each vehicle's cabin volume. Further, testing vehicles for compliance with environmental standards involves three phases, none of which involve engine idling (during which CO emission could be higher) only.

Secondly, CO emissions from vehicles with catalytic converters may exceed the legislated limit. Specific examples are:

• the engine idles from a cold start, with a delay of 1.5-3 minutes before the catalytic converter has warmed up and is operating efficiently (as blood COHb concentration rises most rapidly when first exposed to CO the initial absorption rate would be particularly high³ );

• the condition of the catalytic converter has deteriorated or the engine may require tuning (under ADR 37-00 a catalytic converter is required to operate effectively for 80 000 km or five years, whichever occurs first); and

• for reasons that are unclear, the pollution performance of 4-9-year-old cars has been found to deteriorate faster than that of older cars.¹³

A 1989 US report noted that, even with reduced CO emission limits and catalytic converters, it is still possible to use car exhaust for suicide.¹⁰ If exposure is prolonged, residual CO content would eventually cause death, or suffocation might occur. Also, destruction of the engine management system, by the use of leaded petrol and neglect, or physical disconnection of the engine management system, could increase the CO content, and thus the lethality of this method for suicide.⁸

Further research needs to be undertaken in this technically complex area. A study of the vehicles used for unsuccessful exhaust gas suicide attempts and of other factors contributing to their failure would assist in understanding the relationship between vehicles and exhaust gas suicides. New regulations in Australia have not yet reduced suicides from motor vehicle exhaust gas. There is a need to monitor the situation, especially in the light of the 1997 reduction in CO to 2.1 g/km, and to review the regulations, vehicle design and testing methods.  

Acknowledgments

References

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