Using U.S. national data from 1978 to 1998, we found that county altitude had a significant positive association with overall suicide rates, firearm-related suicide rates, and nonfirearm-related suicides. Altitude was associated with overall suicide rate even after controlling for five potential confounders. The correlation between altitude and suicide could be mitigated by a positive correlation between altitude and all-cause mortality over the same period. On the contrary, we found a significant negative correlation between altitude and all-cause mortality, a finding that highlights the novelty and strength of the observed relationship between altitude and suicide. Using a different methodology, a similar, strong positive correlation between altitude and suicide rates has recently been reported by Kim and colleagues, (2011
) using all counties in the United States as well as all 233 counties in South Korea.
Our previous abstract work on altitude and suicide (Cheng et al., 2005
) was done by comparing mean state altitude with suicide data for entire states and was repeated by others using peak altitude for each state and state capital city elevations (Haws et al., 2009
). Although both studies reported strong correlations (r
0.75 and 0.74, respectively), they were inconclusive. In these studies the mean or highest state altitude (Cheng et al., 2005
; Haws et al., 2009
) or the elevation of the state capital city (Haws et al., 2009
) was used to represent the altitude of the entire state. However, because U.S. states vary greatly in altitude and the foregoing methodology severely minimizes this variation, we considered these initial findings to be of a preliminary nature only. For example, New York varies from sea level to 5344
ft and California from −282 to 14,505
ft. With such heterogeneity in elevation on a state basis, it would be difficult to conclude that elevation might be related to suicide, despite the strong ecologic correlation. Counties vary much less in altitude than an entire state. For this reason, we thought that reexamining this association on a county level would address this limitation.
If there were no link between altitude and suicide, there is little reason why the 50 counties with the highest suicide rates should differ in elevation when compared to the counties with the 50 lowest suicide rates. We found, however, that there was an almost 8-fold difference in altitude in these two groups of counties. When suicides were divided by firearm status (yes or no), the difference in altitude between the 50 counties with the highest and lowest suicide rates was 4.3 and 3.8, respectively. Prior reports of increased suicides in the U.S. Mountain Region (e.g., Colorado) have prompted speculation that the excess is owing to greater access to firearms, increased isolation, or reduced income (CDC, 1997
). Even after controlling for these variables in our analysis, the positive correlation between altitude and suicide still exists, which suggests that the increased suicide rate in the regions with greatest altitude, such as the Mountain Region, may be owing to, at least in part, its altitude per se.
Although a discussion of potential mechanisms is speculative at this juncture, we believe it appropriate to guide further investigation into this novel finding. For example, altitude is a well-known cause of hypoxia, and the greater the elevation, the greater the hypoxia. Chronic hypoxia also is thought to increase mood disturbances, especially in patients with emotional instability (Shukitt and Banderet, 1988
; Nicholas et al., 2000
; Nock et al., 2010
). The relationship between mood and hypoxia is complex, because oxygen therapy, while beneficial to pulmonary function in hypoxic patients with sleep apnea, was found not to improve mood (Yu et al., 1999
Humans have well-known physiologic responses to mild and moderate chronic hypoxia, such as increased 2,3-diphosphoglycerate and a shift to the right in the hemoglobin–oxygen dissociation curve (Winslow, 2007
); but not all people respond equally to hypoxia or increased altitude owing to variations in hemoglobin affinity for oxygen and other mechanisms (Winslow, 2007
). If the mechanism of the suicide–altitude relationship were hypoxia, we would anticipate that there may be increased mood disturbances at high altitude in those with sleep apnea (Peppard et al., 2009
) or moderate or heavy smokers at high altitude.
Future studies may or may not confirm the altitude–suicide association in other parts of the world. Should the association not be present in some other locations with comparable variation in altitude, it is possible that our findings are owing to conditions that are more common in the United States. For example, although obesity rates are rising worldwide, they have been high in the United States for decades (Peppard et al., 2009
). Obesity is known to cause increased hypoxia owing to sleep apnea and thereby may create a mood disturbance (Rigby et al., 2004
); one might anticipate that the altitude–suicide finding might be heightened in obese individuals. Known periodic breathing at high altitude may further exacerbate the effects of sleep apnea and nocturnal hypoxia (West et al., 1986
, Khoo et al., 1996
; Bloch, 2010
A potential limitation regarding the altitude–suicide finding is heterogeneity in altitude within counties. Although the problem is obviously worse when considering entire states (Cheng et al., 2005
), it is a lesser concern even for large counties. However, the consistency of the association across different measures of altitude [i.e., when measured at both the state level (Cheng et al., 2005
; Haws et al., 2009
) and now the county level] suggests that the association is not spurious. We addressed other potential limitations in the analysis (e.g., contribution of Mountain states, exclusion of unreliable data), and the altitude–suicide finding was very robust.
Despite the strong association between suicide and altitude, other factors may be responsible for this association that are directly related to high altitude per se, for example, low barometric pressure (Shukitt et al., 1998
). Many demographic, psychiatric, and sociocultural factors are associated with suicide, and association between high altitude and suicide is speculative. But when other risk factors were considered, the strong association between altitude and suicide was still present in suicides overall and in suicides both with and without firearms. This strong association (r
0.50) is rendered even stronger by the overall negative association between all deaths and altitude (r
In summary, altitude is strongly associated with suicide rates in the United States. This novel finding is not explained by county differences in demographic factors, income, or geographic isolation. Future studies might focus on the individual differences between these high and low altitude areas, both at the biochemical level (e.g., glycolysis, serotonin metabolism, oxygen transport) and the level of the entire organism (e.g., differences in arterial oxygen compared with pulse oximetry, body mass index, sleep apnea, smoking, or behavioral distinctions). Ultimately, this mechanistic search might help clinicians to identify individuals at high altitude who may be amenable to relocation to lower altitude areas, oxygen therapy, or special monitoring and intervention (U.S. Department of Health and Human Services, 2009