Suicide risk increases in the days following exposure to certain air pollutants, according to a recent study.
Bakian AV, Huber RS, Coon H, et al. Acute air pollution exposure and risk of suicide completion. Am J Epidemiol. 2015;181(5):295-303.
Time-stratified case-crossover design
Salt Lake County, Utah residents (N=1,546) who completed suicide between 2000 and 2010
Study Parameters Assessed
The study assessed daily exposure to air pollutants among Salt Lake County residents from January 1, 2000, to December 31, 2010. This was done by looking at data on 4 air pollutants:
- nitrogen dioxide,
- particulate matter with an aerodynamic diameter ≤2.5 μm (PM2.5),
- particulate matter with an aerodynamic diameter ≤10 μm (PM10), and
- sulfur dioxide.
In addition, researchers looked at meteorological data and daily sunlight data for the area.
A heightened risk of suicide following acute exposure to nitrogen dioxide was observed on lag day 3 in the fall [average of the 3 days preceding suicide; odds ratio (OR)=1.20, 95% confidence interval (CI): 1.04, 1.39] and fine particulate matter (diameter ≤2.5 μm) on lag day 2 (day 2 before suicide; OR = 1.05, 95% CI: 1.01, 1.10).
Seasonal variation reflected an increased suicide risk associated with exposure to nitrogen dioxide during the spring/fall transition period (OR = 1.35, 95% CI: 1.09, 1.66) and fine particulate matter in the spring (OR = 1.28, 95% CI: 1.01, 1.61) during cumulative lag 3.
The study included 1,546 case days and 4,297 control days. For males, the largest percentage of suicides occurred in the spring (28%); for females the highest percentage occurred in the fall (30%). Winter saw the lowest percentages for both genders: 22% for each. Increased suicide risk was associated with single-day and cumulative exposure to fine particulate matter (PM2.5) and nitrogen dioxide on and during the 2 to 3 days preceding a suicide. Suicide risk was higher in the spring and the spring/fall transition period after acute exposure to PM2.5. Risk was also higher in both the fall and the spring/fall transition period after acute exposure to nitrogen dioxide.
Air pollution and its impact on human health are of growing concern throughout the world. Recent studies have focused mainly on respiratory and cardiovascular mortality. Historically, research on environmental contributors to suicide risk has been focused on weather. The study reviewed here shows a heightened risk of suicide related to short-term exposure to airborne particulate matter.
Suicide is a serious public health issue worldwide, with multiple precipitating factors (eg, severe mental illness, alcohol/drug abuse, loss of a loved one, exposure to violence, social isolation). Environmental factors in suicide risk have been researched, but until recently little evidence has been available on whether air pollution is related to suicide.
A previous study conducted in Korea found that an increase in fine particulate matter was associated with increased suicide risk, especially when cardiovascular disease was present.1 In addition to fine particulate matter, the publication being reviewed here also considered nitrogen dioxide, sulphur dioxide, and seasonal variation before suicide completion. It also appears to be the first US-based study to have investigated an association between short-term air pollution exposure and heightened suicide rate.
This study reflects a small but growing body of research linking suicide risk to air pollution exposure. The demonstrated association between air pollution and suicide rate adds to the disease burden already associated with air pollution.2,3 These occurrences might be explained through sociological, biological, and now psychological effects. Weather, air pollution, and other climate variables may influence behavior leading up to suicide. These can cause fatigue or cardiorespiratory symptoms, further influencing an individual’s sense of well-being and possibly influencing the decision to end one's life.4
This study suggests air pollution may interact with spring or spring/fall suicide risk factors. In addition to possible meteorological influences, seasonally related comorbid conditions may contribute (eg, mood disorders, pollen allergies). This is in contrast to the prevailing understanding: that wintertime exacerbation in Utah occurs because atmospheric inversions trap air pollutants—principally particulate matter—in a layer of cold air. This is a scenario already associated with poor physical health outcomes.
Many explanations have been proposed to account for the negative associations between air pollutants and suicide risk. Both PM2.5 and nitrogen dioxide are small enough to permeate lung tissue and be absorbed directly into the bloodstream.5 They are potent oxidants, and their inhalation has been shown to reduce respiratory function, leading to decreased oxygen saturation, oxidative stress, and hypoxemia.6,7 Studies of hypobaric hypoxia (low circulating oxygen due to low oxygen air concentration) have shown that exposure can cause an increase in dopamine and tyrosine hydroxylase levels in the brain. Furthermore, decreases in serotonin and tryptophan hydroxylase levels are possible.8 The neurobiology of suicide has implicated decreased levels of serotonin.9
People with depression enhanced by air pollutants may be at increased risk for suicide.
Acute inflammatory effects have been seen with exposure to fine particulate matter and nitrogen dioxide. This may increase suicide risk by altering the functioning of the central nervous system.10 The pathophysiology of depression is linked to inflammation through the release of proinflammatory cytokines.11 People with depression enhanced by air pollutants may be at increased risk for suicide. Similar to the hypoxia studies mentioned, exposure to PM2.5 has been shown to exacerbate physical health conditions such as cardiovascular and respiratory illness,12 which can trigger depression and may ultimately increase a person’s risk of suicide.
Nitrogen dioxide and particulate matter may have similar biological mechanisms with regard to suicide, but particulate matter has received more attention than nitrogen dioxide; however, both have been linked to emergency room visits for depression and symptoms of depression.13,14 In addition, outdoor PM2.5 and nitrogen dioxide concentrations are both products of industrial and domestic combustion, and they therefore have been found to be highly correlated.15 This can make it challenging to separate the contributions of individual pollutants.16 Furthermore, correlated air pollutants can have confounding17 or modifying18 effects. Still, controlling for multiple pollutants in an analysis has not been found to significantly affect each pollutant's risk estimate.19,20
Nitrogen dioxide is one of many pollutants found in ambient air. Because it is present in motor vehicle emissions, its presence in outdoor air in urban locations is makes it a strong indicator of vehicle emissions, which include other pollutants as well. Nitrogen dioxide is also a precursor for other secondary air pollutants,21 and that creates concern about potential synergistic effects.
This publication does not address potential epigenetic effects and the plausible negative impacts on the developing central nervous systems (CNS) of the young and unborn since the effects of air pollution on the CNS are not broadly recognized. Furthermore outdoor pollution has become a global public health problem, especially in megacities and in underdeveloped countries.
Because fine and ultrafine particulate matter are small enough to reach the brain, some research has focused on how they might be associated with stroke and depression in adults. In children, research is pointing to systemic inflammation, immune system dysregulation, neuroinflammation, and brain oxidative stress—as well as the hallmarks of Alzheimer’s disease and Parkinson’s disease.22
Research on air pollution and CNS effects in children is exceedingly important, given the lifelong effects of early neurological damage. It is advisable that primary healthcare providers include a complete prenatal and postnatal assessment of environmental and occupational exposures to indoor and outdoor toxic hazards. Additional measures should be taken to prevent or reduce further exposures.22
This call to action is particularly significant in light of a 2014 study indicating that “early life exposure to cigarette smoke is associated with increased risk for depression in midlife.”23
On a more positive note, there is ample evidence that automobile emissions systems designed to reduce air pollution have resulted in a secondary benefit of reducing carbon monoxide (CO) poisoning suicides from automobile exhaust fumes.24
Limitations of the study under review include that the findings may not apply to other parts of Utah or to the United States as a whole. However, the data found consistent overall positive associations between air pollution and suicide rate, emphasizing the importance of prevention through reduced exposure.