Trends in solids/liquids poisoning suicide rates in Taiwan: a test of the substitution hypothesis

It is noteworthy that dominant suicide methods differ greatly across countries, particularly between Asian and Western countries. Poisoning with pesticides was the most common suicide method used by Asian people living in rural areas (notably China, Sri Lanka, Taiwan and India) [16]. Restricting access to pesticides was strongly suggested by the World Health Organization to be a major suicide prevention strategy for Asian countries [17]. However, few studies from Asian countries have investigated whether there was method substitution once access to pesticides was restricted.

In Taiwan, poisoning with solid/liquid substances (mainly pesticides) and hanging were the two most common suicide methods, and were responsible for about 90% of all suicide deaths before 1990 [18]. The solids/liquids suicide rate dramatically decreased during the 1980s, due to the decline in the agricultural population and a series of measures launched by the government to restrict the availability of pesticides in the general population [19]. However, little is known about whether method substitution occurred in Taiwan after restricting access to pesticides.

The aim of the study was to investigate whether the reduction in the solids/liquids poisoning suicide rate in 1983-1993 after the launching of pesticide restriction interventions in Taiwan was accompanied with an increase in the suicide rate using other methods (method substitution).

Figure 1 represents the age-adjusted method-specific suicide trends with 3-year moving average for both sexes, from 1971 through 1993 in Taiwan. During the study periods, solids/liquids poisoning and hanging were the two major suicide methods. The solids/liquids suicide rates markedly and steadily decreased from 12.6 per 100,000 to 3.7 in males and from 10.0 to 2.6 in females between 1983 and 1993. Around the same time, there was no obvious rise in the suicide rates for hanging or other methods in both sexes. Further analysis was done by examining method-specific suicide trend by age and sex from 1971-1993 and found that limited substitution seemed to takes place among males aged 25-44 years (Figure 2).

In further analysis of the age-adjusted poisoning suicide rates by pesticides, medications, and other substances for the years 1987 and 1992, we found that as the male pesticide suicide rates decreased from 5.6 to 2.1 per 100,000 from 1987 to 1992, the medication suicide rates and other substance suicide rates also decreased, from 1.4 to 1 per 100,000 and from 1 to 0.4 per 100,000, respectively. For females, the age-adjusted suicide rates by pesticides, medications, and other substances also decreased from 1987 to 1992: pesticide suicide rates decreased from 3.6 to 1.7 per 100,000, and medication suicide rates decreased 1.3 to 0.6 per 100,000; other substance suicide rates decreased from 1 to 0.5 per 100,000.

Our findings indicated no compensatory rise in suicide rates by hanging or other methods from 1983 to 1993 in Taiwan, accompanying the marked decrease in solids/liquids poisoning suicide rates. The same pattern of change occurred in different sex and age groups.

Several previous studies illustrated that restricting access to lethal methods can reduce suicide rates [1–11]. The most often cited example was Kreitman's study [1], which demonstrated a reduction not only in gas-specific suicide rates, but also in the overall suicide rates because of the lack of a compensatory rise in the use of other methods. However, Gunnell et al., who offered the criticism that those analyses have either failed to examine trends in method-specific suicide rates or have not assessed age- and gender-specific effects, reanalyzed the suicide trends in England and Wales in 1950-75, and found that the effects of these reductions on overall suicide rates due to coal gas detoxification were partially offset by rises in drug overdose deaths in women and younger men [13].

In fact, some other studies showed that method substitution did occur in some specific subgroups and with certain methods, as summarized in Table 1[3, 8, 12–15]. Among those studies, method substitution seemed to be most likely found in young people [14, 15] or in males [3, 8, 12]. In addition, as seen in Table 1, we also found that non-violent methods (e.g., domestic gas) were often replaced by non-violent methods (e.g., car exhaust or drug overdose) [3, 12, 13] and that violent methods (e.g., firearms) were often replaced by violent methods (e.g., jumping or hanging) [8, 14].

If suicide rates in Taiwan followed the substitution hypothesis, we would expect a compensatory increase in suicide rates by overdose, domestic gas and other gas in specific demographic subgroups. Our analyses, however, did not find such results. Why was there no method substitution in Taiwan? And what are the factors affecting the occurrence of substitution?

Three possible factors proposed by Gunnell et al. [13], i.e., the accessibility to, and the acceptability and popularity of the lethal methods were found to have profound effects on method substitution.

As to accessibility, there were nearly no limitations on the availability of medications, since people in Taiwan were free to visit physicians and to buy medications in the drugstore. Similarly, domestic gas was also available to nearly all people, because it was the main heating source for cooking and bathing in every household. Also, car ownership had increased annually due to the 1983-1993 economic development in Taiwan.

In terms of acceptability, there was no stigma to using poisoning with medications, poisons and gas as suicide methods in Taiwan society. However, there was still no method substitution in Taiwan. This may be due to the lack of popularity of other suicide methods during the period 1983-1993 in Taiwan.

The most popular suicide methods in Taiwan were poisoning with solids/liquids (mainly pesticides) and hanging, which constituted 90% of all suicide deaths in the period 1983-1993 [18]. The other suicide methods popular in Western countries, such as car exhaust, medicine overdose, and gassing, contributed little to suicide deaths in Taiwan. Therefore, the lack of popularity of other violent suicide methods similar to pesticide overdosing before 1993 may be one of the possible explanations for the phenomenon of no method substitution in Taiwan.

There were some limitations to our study. First, the E-codes of the ICD-8 and ICD-9, used in Taiwan during the study periods, only included three-digits. The code E950 (and its analogous counterpart E980) is a broad category that includes suicide from all types of solid and liquid substances, such as pesticides, prescriptions and other poisons. However, the data from the years 1987 and 1992 show that pesticides were the major ingested solid/liquid substances causing suicide death in Taiwan. In addition, no compensatory rise in suicide rates by medications and other solid/liquid substances was found, as the pesticide suicide rates decreased markedly from 1987 to 1992. In fact, the sex-specific suicide rates by poisoning with pesticides, medications and other substances all decreased from 1987 to 1992.

Second, suicide death is thought to be a complex phenomenon involving multiple factors. The influence of many other factors, such as mental health and the socio-economic environment on sex-, age- and method-specific suicide rates was not considered in this study. Further study is needed to test whether changes in psycho-socio-economic conditions would have differential effects on the method-specific suicide rates among particular subpopulations.

Thirdly, one may wonder why did authors not include more recent years for analysis? In fact, the suicide methods changed much in the recent decade in Taiwan. In a previous study, Lin and Lu reported [21], during the 1970's and 1990's, the two leading suicide methods in Taiwan were poisoning by solids/liquids and hanging. These two methods were responsible for about 90% of all suicide deaths before 1990. In the contemporary years of 1991-2005, nearly all method-specific suicide rates seemed to increase in both sexes. However, the distribution of suicide methods changed much. The contribution of the traditional suicide methods in Taiwan (i.e., hanging and poisoning by solids/liquids) decreased, while that of the new methods (i.e., jumping from heights and poisoning with other gases) markedly increased. In this study, we focused on the testing of substitution hypothesis. So, we chose the period 1983-1993, when the suicide methods were relatively simple, to test the substitution hypothesis.

In general, no method substitution was found along with the reduction in solids/liquids suicide rates in Taiwan. Our study results have also added the evidence that restricting access to methods maybe a promising strategy in preventing suicide, particularly in those countries where the "target method" has been found to contribute greatly to the suicide rates.