Relationship between Vehicle Emissions Laws and Incidence of Suicide by Motor Vehicle Exhaust Gas in Australia, 2001–06: An Ecological Analysis

In an ecological study, David Studdert and colleagues show that areas of Australia with fewer vehicles pre-dating stringent carbon monoxide emission laws have lower rates of suicide due to asphyxiation by motor vehicle exhaust gas.Background: Globally, suicide accounts for 5.2% of deaths among persons aged 15 to 44 years and its incidence is rising. In Australia, suicide rates peaked in 1997 and have been declining since. A substantial part of that decline stems from a plunge in suicides by one particular method: asphyxiation by motor vehicle exhaust gas (MVEG). Although MVEG remains the second most common method of suicide in Australia, its incidence decreased by nearly 70% in the decade to 2006. The extent to which this phenomenon has been driven by national laws in 1986 and 1999 that lowered permissible levels of carbon monoxide (CO) emissions is unknown. The objective of this ecological study was to test the relationship by investigating whether areas of Australia with fewer noxious vehicles per capita experienced lower rates of MVEG suicide. Methods and Findings: We merged data on MVEG suicides in Australia (2001–06) with data on the number and age of vehicles in the national fleet, as well as socio-demographic data from the national census. Poisson regression was used to analyse the relationship between the incidence of suicide within two levels of geographical area—postcodes and statistical subdivisions (SSDs)—and the population density of pre-1986 and pre-1999 passenger vehicles in those areas. (There was a mean population of 8,302 persons per postcode in the study dataset and 87,413 persons per SSD.) The annual incidence of MVEG suicides nationwide decreased by 57% (from 2.6 per 100,000 in 2001 to 1.1 in 2006) during the study period; the population density of pre-1986 and pre-1999 vehicles decreased by 55% (from 14.2 per 100 persons in 2001 to 6.4 in 2006) and 26% (from 44.5 per 100 persons in 2001 to 32.9 in 2006), respectively. Area-level regression analysis showed that the suicide rates were significantly and positively correlated with the presence of older vehicles. A percentage point decrease in the population density of pre-1986 vehicles was associated with a 6% decrease (rate ratio [RR] = 1.06; 95% confidence interval [CI] 1.05–1.08) in the incidence of MVEG suicide within postcode areas; a percentage point decrease in the population density of pre-1999 vehicles was associated with a 3% decrease (RR = 1.03; 95% CI 1.02–1.04) in the incidence of MVEG suicide. Conclusions: Areas of Australia with fewer vehicles predating stringent CO emission laws experience lower rates of MVEG suicide. Although those emission laws were introduced primarily for environmental reasons, countries that lack them may miss the benefits of a serendipitous suicide prevention strategy. : Please see later in the article for the Editors' Summary Background: Suicide (self-inflicted death) is a major, preventable public-health problem. About 1 million people die each year from suicide and about 20 times as many people attempt suicide. Globally, suicide rates have increased by nearly a half over the past 45 years and suicide is now among the three leading causes of death in people aged 15–44 years. Within this age group, 1 in 20 deaths is a suicide. Most people who commit suicide have a mental illness, usually depression or substance abuse, but suicide can also be triggered by a stressful event such as losing a partner. Often warning signs are present—a person who talks about killing themselves must always be taken seriously. Adequate prevention and treatment of mental illness and interventions that teach young people coping skills and improve their self-esteem have shown promise in reducing suicide rates, as have strategies (for example, restrictions on the sale of pain killers) that reduce access to common methods of suicide. Why Was This Study Done?: In Australia, the suicide rate has been declining since 1997 when a record 2,722 suicides occurred. Fewer suicides by asphyxiation (oxygen deprivation) by motor vehicle gas exhaust (MVEG) account for much of this decline. MVEG contains carbon monoxide, a toxic gas that blocks oxygen transport around the body. Although MVEG suicide is still the second most common means of suicide in Australia, its incidence has dropped by two-thirds since 1997 but why? One possibility is that national laws passed in 1986 and 1999 that lowered the permissible level of carbon monoxide in vehicle exhaust for environmental reasons have driven the decline in MVEG suicides. Evidence from other countries suggests that this might be the case but no-one has directly investigated the relationship between MVEG suicide and the use of vehicles with reduced carbon monoxide emissions. In this ecological study (a study in which the effect of an intervention is studied on groups of people rather than on individuals), the researchers ask whether the number of pre-1986 and pre-1999 vehicles within particular geographic areas in Australia is correlated with the rates of MVEG suicide in those areas between 2001 and 2006. What Did the Researchers Do and Find?: The researchers obtained data on MVEG suicides from the Australian National Coroners Information System and data on the number and age of vehicles on the road from the Australian Bureau of Statistics. MVEG suicides dropped from 498 in 2001 to 231 in 2006, they report, and 28% of passenger vehicles registered in Australia were made before 1986 in 2001 but only 12% in 2006; the percentage of registered vehicles made before 1999 fell from 89% to 60% over the same period. The researchers then used a statistical technique called Poisson regression to analyze the relationship within postcode areas between the incidence of MVEG suicide and the presence of pre-1986 and pre-1999 vehicles. This analysis showed that in areas where older vehicles were more numerous there were more MVEG suicides (a positive correlation). Specifically, the researchers calculate that if the proportion of pre-1986 vehicles on the road in Australia had stayed at 2001 levels throughout their study period, 621 extra MVEG suicides would have occurred in the country over that time. What Do These Findings Mean?: These findings show that in areas of Australia that had fewer vehicles on the road predating stringent vehicle emission laws, there were lower rates of MVEG suicide between 2001 and 2006. Unfortunately, this study cannot provide any information on the actual age of vehicles used in MVEG suicides or on the relationship between vehicle age and attempted MVEG suicides. It also cannot reveal whether those areas that had the sharpest decreases in the density of older vehicles had the sharpest decreases in suicide rates because very few suicides occurred in most postcodes during the study. Most importantly, the design of this study means that the researchers cannot discount the possibility that the changes in Australia's emission laws have steered people towards other methods of taking their own lives. Nevertheless, the findings of this study suggest that the introduction of stringent vehicle emission laws for environmental reasons might, serendipitously, be a worthwhile long-term suicide prevention strategy in countries where MVEG suicide is common. Additional Information: Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1000210.