Selenium rectifiers had a shorter lifespan than desired. In the early stage of failure they produce a modest amount of sweet-smelling gas, sometimes described as '"sickly sweet'". At that point the rectification properties are almost totally gone, allowing reverse voltage to leak through the rectifier. During catastrophic failure they produce significant quantities of malodorous and highly toxic [[hydrogen selenide]]<ref>{{Cite journal |date=1950-08-22 |title=Constitution and mechanism of the selenium rectifier photocell |url=https://royalsocietypublishing.org/doi/10.1098/rspa.1950.0112 |journal=Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences |language=en |volume=202 |issue=1071 |pages=449–466 |doi=10.1098/rspa.1950.0112 |bibcode=1950RSPSA.202..449P |issn=0080-4630 |last1=Preston |first1=J. S. |s2cid=93164294 }}</ref><ref>{{Cite web |last= |year=1978 |title=Occupational Health Guideline for Hydrogen Selenide |url=https://www.cdc.gov/niosh/docs/81-123/pdfs/0336.pdf |access-date=2022-10-01 |website=cdc.gov}}</ref><ref>{{Cite web |last=PubChem |title=Hydrogen selenide |url=https://pubchem.ncbi.nlm.nih.gov/compound/533 |access-date=2022-10-01 |website=pubchem.ncbi.nlm.nih.gov |language=en}}</ref> that let the repair technician know what the problem was. By far the most common failure mode was a progressive increase in forward resistance, increasing forward [[voltage drop]] and reducing the rectifier's efficiency. During the 1960s they began to be superseded by [[silicon rectifiers]], which exhibited lower forward voltage drop, lower cost, and higher reliability.<ref>[http://pubs.usgs.gov/of/2003/of03-018/of03-018.pdf Selenium], U.S. Department of the Interior].</ref>
