A meta-analysis has found that sperm concentration has declined more than 50 percent in less than 40 years among men from North America, Europe and Australia.

The meta-regression analysis on 185 studies between 1973 and 2011 revealed a 52.4 percent decline in sperm concentration, and a 59.3 percent decline in total sperm count, but there was no significant decline was seen in South America, Asia and Africa, where far fewer studies have been conducted.

The study also indicates the rate of decline among Western men is not decreasing: the slope was steep and significant even when analysis was restricted to studies with sample collection between 1996 and 2011.

While declines in sperm count have been reported for years, the question has remained controversial because of limitations in past studies.

The current study uses a broader scope and rigorous meta-regression methods to conservatively address the reliability of study estimates, and control for factors that might help explain the decline such as age, abstinence time, and selection of the study population.

The findings have important public health implications.

First, the data demonstrates that the proportion of men with sperm counts below the threshold for subfertility or infertility is increasing. Moreover, given the findings from recent studies that reduced sperm count is related to increased morbidity and mortality, the ongoing decline points to serious risks to male fertility and health.

The study did not examine causes of the observed declines, but sperm count has previously been plausibly associated with environmental and lifestyle influences, including prenatal chemical exposure, adult pesticide exposure, smoking, stress and obesity.

Therefore, sperm count may sensitively reflect the impact of the modern environment on male health across the lifespan and serve as a ‘canary in the coal mine’ signalling broader risks to male health.