Bioclimatic factors have been suggested to be potential mediators of the seasonal variation in suicide, though this concept is controversial. Some researchers have documented a positive association between sunshine/temperature/humidity and suicide [20,21,22,23,24,25,26], while others dispute this relationship [27,28,29,30]. In addition, a few studies concluded that a positive association between climatic factors and seasonal variation of suicide was present only for suicide by violent methods [14,31].

Petriduo et al. [32] suggested that sunlight may act as a trigger of suicide. In addition, suicide rates are greater in rural areas than in urban areas [2,4,33,34] and higher among outdoor workers compared to indoor workers [35]. Some empirical findings support the notion that the intensity of sunlight may play a role in the triggering of suicide and therefore provide a potential link to the seasonal variation in suicide. Hiltunen et al. reported the association between increased suicide mortality and the period with the longest day length (which was between May and July) i.e., late spring/early summer [36]. Another study in Greenland reported a similar pattern. However, both studies suggested the role of latitude and other signals besides the variation in daylight, as the suicide peak of the northern area of Finland (Oulu) was delayed when compared to the southern area (Helsinki) and the strength of the suicide peak was more pronounced at higher latitudes [36,37]. A recent analysis of data from Finland suggested a correlation between solar radiation and suicide mortality [38] but other studies have also suggested that seasonal suicide peak in spring occurs significantly later than the interval of change in day length [39,40,41]. Furthermore, Papadopoulos et al. [42] hypothesized that a time lag exists for the effect of solar radiance on suicidality. In summary, seasonal changes in sunlight seem unlikely to fully account for the seasonal variation in suicide.

With regard to temperature, a study performed by Volpe et al. [25] found that suicide rates in Brazil not only showed a higher peak in December and January than the rest of the year, but were also significantly correlated with increasing temperature. In addition, Kim et al. [15] reported a 1.4% increase in suicide when temperature goes up by 1 degree Celsius. Temperature could either be a marker of seasonal change, or the mediator of it. In addition, specific meteorological conditions such as temperature and thunderstorm for the preceding day could contribute to increased risk of suicide in individuals [26].

Precipitation (rainfall and snowfall) is another climatic factor that shows seasonal variation and has thus been postulated to possibly be predictively associated with seasonality of suicide. When Ajdacic-Gross et al. [28] modeled monthly data on suicide and precipitation in Switzerland precipitation did not show any noteworthy effects on suicide frequencies. Lin et al. also examined the association between monthly suicide rates and climatic influences including atmospheric pressure, temperature, sunshine, humidity, and rainfall in Taiwan; however, they only found evidence of an association of temperature with seasonality, but reported no significant association between rainfall and seasonal peaks of suicide in spring/early summer [43].

Chew and McCleary [4] comprehensively compared the seasonal variation of suicide across 28 nations and found well replicated seasonal spring peaks in suicide rates from the various nations regardless of the location of the countries. They also observed wide cross-sectional variation in degree of suicide seasonality. For instance, when comparing Canada to Portugal they demonstrated a narrow range of seasonal fluctuation in Canada (ratio of average spring to average winter = 1.08) versus wide fluctuation of seasonal suicide rates in Portugal (ratio of average spring to average winter = 1.70) implicating a more prominent seasonal spring peak in Portugal.

Consistent with the pattern in the northern hemisphere, Flisher et al. [44] reported a mirror image spring or summer peak of suicide and a trough in fall in South Africa, especially for less urbanized subpopulations. Similarly, studies in Australia [24] are concordant with studies conducted in the Northern Hemisphere in Europe [1,10,11,32,45,46,47,48] and Asia [18,49,50], identifying a seasonal spring suicide peak.

Allergy has been previously linked to suicide [13]. The seasonality of suicide has been shown to co-occur with the seasonal peaks in ambient pollen concentration during spring (i.e., tree pollen), summer (i.e., grass pollen), and fall (i.e., ragweed) [19]. Pollens are aeroallergens and are capable of inducing an allergic inflammatory reaction when they reach the intranasal mucosa of susceptible individuals. The inflammatory reaction induced by aeroallergens involves the production of Th2 cytokines which, in animal models, have been associated with increased anxiety-like behavior, reduced social interaction [51] and aggressive behavior [52] all of which can be considered as endophenotypes for suicidal behavior [53]. Furthermore, the seasonal peak in aeroallergens resulting in the concomitant worsening of allergy symptoms could (via inflammatory mediators of worsening allergy symptoms) potentially worsens depressive symptoms, anxiety and impulsivity in mood disorder patients, resulting in exacerbated risk of suicidal behavior. Consistent with this notion, Manalai et al. [54] recently reported that in bipolar patients pollen-specific IgE positivity and worsening of allergy symptoms are associated with worsening of depression scores during exposure to aeroallergens. In addition, changes in allergy and anxiety (anxiety representing a potentially independent suicide risk factor) in patients with mood disorders exposed to seasonal peaks of aeroallergens were observed to be correlated [54]. In essence, the current available evidence makes seasonal fluctuation of aeroallergens a possible factor involved in the underlying mechanisms responsible for seasonality of suicide. This is particularly important from a neuroimmune perspective, considering a previous study showed an increased gene expression for cytokines involved in allergic reactions in the orbitofrontal cortex (a region previously implicated histopathologically with suicide) in victims of suicide [53].

The human immunodeficiency virus (HIV) has been associated with suicide [55] but no seasonal pattern has been reported in relation to HIV-related suicide rates neither has HIV been known to manifest a seasonal pattern of infectivity. The influenza virus, on the other hand, has a seasonal pattern of infectivity. However the only report of an association of influenza with suicidal behavior [17] did not include an evaluation of seasonality effect on suicide. More studies on the association of seasonal viruses and suicide are needed.

Air pollutants have been correlated with rates of visits to the emergency room as well as inpatient admission rates of patients with mental illness [56]. Recently Kim et al. [16] found that the rate of completed suicide in the Republic of Korea was elevated when there was an increase in the ambient particulate matter two days prior to the day of suicide. The two aforementioned studies did not take into consideration the impact of the season on elevation of suicide risk. Szyszkowicz [57], however, carried out an analysis of data on air pollution effect on emergency room (ER) visit for worsening depression by season and found that the highest percentage of depression-related ER visits were during periods of increased concentration of ambient particulate matter during the cold season and the finding was limited to only females. However, the findings by Szyszkowicz should be interpreted with caution (in terms of elucidation of the seasonality effects of air pollutants on depression), since the analysis did not include an assessment of an interaction between season and pollutant but rather an analysis by season was carried out. It therefore appears that the literature on the potential contribution of pollutants to the seasonality of suicide is sparse and no generalization can be made at this time.

Several studies have found associations between age and seasonal variation in suicide. Maes et al. [2] reported that the suicide rate among younger people was increased in spring (i.e., March and April), whereas the rate within older adults was raised in late summer (i.e., August). However, Lahti et al. [66] observed a suicide peak in fall among adolescents, particularly for those dying by shooting. Furthermore, McCleary et al. [67] documented that a suicide peak was observed in younger aged individuals in winter and fall, while suicide among the very old was elevated towards the summer period. The inconsistency between studies may reflect methodological or environmental differences between studies and at the moment, no conclusion can be drawn.

Although seasonality of suicide completion is seen both in men and women, the seasonal patterns differ between genders. For example, only a single spring peak is found in men, while two peaks in spring and fall have been reported in women [4,33,34,40,68]. In England, middle aged women who had school-aged children were more likely to commit suicide in fall, which was the beginning of the school year [4,69]. Though speculative, it may be that a sudden reduction in the duration of direct contact with a dependent represents a type of suicide risk in these women. Gender effect on seasonality of suicide was also noted in Hungary where a steadily increasing prescription rate for antidepressants was associated with a decrease in national suicide rate but significantly decreased seasonality of suicide only in males [70]. The suicide peak in spring has been considered to be a consequence of seasonal occurrence of depression-related suicides and the decreased seasonality of suicide in this Hungarian study was suggested to be a marker of lowering depression-related suicides (especially among men) as a result of increased antidepressant utilization in the population [70].

Seasonality of suicide attempts is also shown to be associated with gender [71]. Studies performed in Scotland and in Oxford revealed a seasonal variation of female suicide attempts with increased rates during summer and decreased rates in winter, but no significant seasonal variation of male suicide attempts was found [72,73]. In addition, the results of the WHO/EURO Multicenter study on Parasuicides indicated that the seasonal pattern of suicide attempts in women showed a peak in spring and nadir in winter, but no significant variation of suicide attempts was observed within the male subpopulation [74]. However, there have been negative reports as well. Mergl et al. analyzed suicide attempts in Nuremberg and Wuerzburg from 2000 to 2004 and they failed to confirm the significant gender difference in seasonality of suicide attempts [71]. Kreitman et al. also reported no considerable gender difference in seasonality of suicide attempts in the U.S. [75].

While several studies have reported season of birth effect on suicide or suicidal behavior [7,76,77,78], there is lack of evidence to indicate an association between month of birth and seasonality of suicide completion. Dome et al. [76] found a significantly increased risk of suicide completion among those individuals who were born in spring and summer, however, this study did not show any relationship between season of birth and seasonal variation of suicide. Another study which evaluated the effect of birth month on suicidal behavior in Western Australia reported a notable spring peak of deliberate self-harm and a significantly increased birth in spring within the deliberate self-harm group [7]. However, no season-of-birth effect was observed in relation to completed suicide in the study.

Socioeconomic status can affect suicide rates. Social discrepancy, disputes, socio-economic gradient (urban-rural income gradient, etc.), divorce and resulting single parent family environment can be related with seasonality of suicide. The majority of research findings indicated that the seasonal spring peaks are greater in rural areas compared to urban areas [2,4,79]. In particular, Micciolo et al. [79] evaluated the seasonality of suicide in Italy from 1969 to 1984 and found the suicide peaks in spring to be more notable in rural areas than in urban areas, although the suicide rates was higher in urban regions. A review by Christodoulou et al. [80] suggested that this phenomenon might plausibly be related to intensity of seasonal activities such as agricultural work in the rural areas. In fact, Chew and McCleary [4] reported that the spring peak of suicide is relatively larger in agricultural countries compared to industrial countries. They also found that the larger amount of agricultural work is significantly correlated with the greater spring peak of suicide. Ajdacic-Gross et al. [81] further posited that as the traditional rural society is disappearing with industrialization, the seasonal variation of suicides might be attenuated.

In addition, seasonality of suicide has been shown to be related to occupational differences. The agricultural and construction sectors usually have intense activity from spring to fall. Näyhä [40] found that suicide committed by people who served in technical, administrative, and service work (i.e., modern occupations) usually peaked in late fall, while people who engaged in traditional occupations (e.g., agriculture, transport, or manufacturing work) showed seasonal peaks of suicide in spring/ summer. Koskinen et al. [35] also examined seasonality of suicide in different occupations including farmers, forest workers, construction, and indoor workers. They documented that spring peak and winter trough of suicide pattern was observed in groups of farmers and forest workers. On the contrary, a significant summer nadir was shown within indoor workers. Moreover, in their sub-group analyses by suicide methods, 90.5% of farmers used violent methods, followed by forest workers (79.1%), construction (73.2%), and indoor workers (69.2%), indicating violent suicides decreased among indoor workers. Considering suicides by violent methods show remarkable peaks in spring [2,43,47], it is plausible to expect seasonal spring peaks with people who are more likely to spend time in outdoor settings [82]. Migrant workers can be exposed to higher mental distress and suicide risk as dramatically depicted in the series of attempted or committed suicides in Foxconn production facilities in China between Jan and May 2010 [83,84,85]. However, seasonal variation of suicide in migrant populations needs to be further studied.

These findings seem to indicate that people who are more exposed to the outdoor environment have a greater seasonal spring peak in suicide—thus, suggesting that factors driving seasonality may be more abundant in the outdoor environment. For example, increased seasonal work related-stress in farmers and increased exposure to outdoor physico-chemico-biological factors such as day length, light, temperature, pollution, pathogens or allergens may contribute to more ample seasonal suicide peaks.