Merzenich et al: BMC Public Health 2010, 10:24 http://www.biomedcentral.com/1471-2458/10/24 D E B A T E Open Access DDebeatcereasing sperm quality: a global problem? Hiltrud Merzenich*, Hajo Zeeb and Maria Blettner Abstract Background: Carlsen and coworkers (1992) reviewed 61 heterogeneous observational studies on semen quality published between 1938 and 1990. This review indicates that mean sperm density decreased significantly between 1940 and 1990. An extended meta-analysis with 101 studies confirmed a decline in sperm density for the period from 1934 to 1996 (2000). The key message of the meta-analyses is that sperm counts have decreased globally by about 50% over the past decades. This assessment has been questioned. Discussion: A major methodological problem of the meta-analysis is the use of data collected in different countries, at different times, on different populations and with different methods of semen analysis. Furthermore, the results of studies concerning semen analysis are frequently biased e.g. by selection criteria of volunteers. In most studies on human semen characteristics the populations under study are insufficiently defined and the study participants are not a representative population sample. The incidence of testicular cancer has increased in Caucasian men worldwide. The investigation of common risk factors for male reproductive disorders requires well designed epidemiological studies and the collection of individual data. Summary: Former meta-analyses of sperm count data show a global downward trend. This conclusion should be interpreted with caution. The included studies are of great heterogeneity due to geographical and/or ethnical variation, different study designs and different methodological standards. Population-based prospective studies are needed to investigate secular trends in male reproductive disorders. Background Analyses of sperm count data suggest a global downward trend but the results are inconclusive. An increase in male reproductive disorders like cryptorchidism and tes- ticular cancer raise the question of common risk factors. Data on testicular cancer trends are usually based on population-based cancer registries with good validity. The corresponding data from fertility studies need care- ful consideration in terms of methods and results. Carlsen et al. [1] reviewed studies on human sperm quality. Publications were excluded if they included men from infertile couples or those referred for oligozoo- spermia or some genital abnormality. Furthermore, pub- lications were excluded if they included men selected for either high or low sperm count and if counting of sperm cells had been performed with non-manual methods. A total of 61 studies has been included covering results published between 1938 and 1990 without any geograph- ical restriction. The data were analyzed by linear regres- * Correspondence: merzenich@imbei.uni-mainz.de 1Institute of Medical Biostatistics, Epidemiology and Informatics, University Medical Center of the Johannes Gutenberg University, Mainz sion in which the weight of each study was dependent on the number of men included in the study. The results of this meta-analysis indicate that mean sperm density decreased from 113 × 106/ml in 1940 to 66 × 106/ml in 1990, and seminal volume decreased from 3.40 ml to 2.75 ml during the same period. These results caused consid- erable attention. Swan et al. [2] excluded from Carlsens' meta-analysis three non-English-language studies and two studies that included men who conceived only after an infertility work up. In total, 56 studies were reanalysed covering results published between 1938 and 1990. The observed decrease in sperm quality was confirmed. In an extended analysis [3] studies published between 1934 and 1996 were considered which meet the eligibility criteria of Carlsen et al. [1]. In total 19 new studies were found. Furthermore, the authors conducted a further search for the 60-year period 1930-1990 and identified 28 additional eligible studies. Two studies with less than 10 subjects were excluded from the original Carlsen studies. In total, 101 studies were analysed. According to Swan et al. the extended meta-analysis supported a decline in sperm density for the period from 1934 to 1996 [3]. The sperm © 2010 Merzenich et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Com- mons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduc- tion in any medium, provided the original work is properly cited.