Suicide rates don’t go up in the winter. They spike in the spring.

Prevailing cultural wisdom tells us that most suicides occur during the cold of winter, during the holiday season when people might be alone or depressed.

But data suggests that isn’t right: For the past 16 years, suicide rates have been highest in late spring and summer, although there is evidence this might be changing.

As you can see above, the number of suicides per day don’t fluctuate dramatically from month to month, but there is a steady uptick starting in late April/early May before rates steadily drop off again in October.

But despite patterns that show seasonal variation in suicide rates over time, the role that underlying environmental and demographic factors play in contributing to higher levels of suicide is largely not understood.

Research exploring the seasonal variation in suicide goes as far back as 1825, but there is remarkably little consensus on the factors responsible for triggering the annual spikes in suicide rates.

There are several theories that attempt to explain the seasonal variation in suicide rates — one focuses on the increase in daylight and another on additional pollutants in the air. But few environmental explanations have gained as much traction as the idea that allergens can cause violent, suicidal behavior.

Some researchers have argued that higher rates of suicide correlate with seasonal peaks in ambient pollen concentrations. For instance, in spring there is a high concentration of tree pollen in the air, in summer, grass pollen, and in the fall, ragweed. Several studies have linked the inflammatory reactions triggered by allergens to increased anxiety, reduced social interaction, and aggressiveness, all of which are potential warning signs of suicidal behavior.

Still, this research remains somewhat inconclusive, and far from a definitive answer on why suicide rates increase in the spring, as the relationship between seasonal allergens and its role in exacerbating mental states is not fully understood.

While the exact underlying reasons for why suicide rates increase during spring are disputed, the majority of research does agree on one thing: Rural areas are disproportionately affected by spring suicides compared with their urban counterparts.

Researchers have found that predominantly agricultural countries have a higher rate of spring suicides than industrial countries, and intense outdoor labor correlates with a greater spring peak in suicide. These findings have led many researchers to conclude that seasonal factors like day length, temperature, and allergens may be more damaging in a predominantly outdoor environment given their increased prevalence.

But what’s particularly interesting about this theory of a rural/urban divide in suicide seasonality can be seen in new research that focuses on what seasonal suicide rates look like in predominantly urban areas.

Some researchers, such as Ajdacic-Gross et al., have argued that as we shift from primarily rural economies to highly industrialized societies, seasonal variation in suicide rates flatten over time. But researcher Simo Näyhä found that instead of seasonal variation smoothing out, a second peak in autumn is emerging for people who work in more modern office jobs.

And this observation definitely appears to be true for the US in recent years. In September 2014, the US recorded its highest number of suicide deaths of any month in the past 16 years, with 4,049 reported suicides. The rate of suicides in September has steadily crept upward since 2010 (although 2015 did reporter fewer suicides—3,598 deaths—than the 2014 high).

Additionally, the latest Centers for Disease Control and Prevention report on suicide rates found that women experienced a greater percentage increase in the number of suicides from 1999 to 2014 than men (even though men still outnumber women in total number of deaths by suicide).

If you are considering suicide, please seek help through the national suicide prevention lifeline at 1-800-273-8255.