Gas filter have no definite duration in practice. Durations given in EN standards are applicaple only to tests carried out in a Lab at well defined conditions. It is common practice to consider the filter worn out when one can smell the odour of the gas in the inhaled air. Not all gases however have an odour threashold sufficiently low to permit the user to smell the gas before the concentration is high enough to be dangerous to health. The Guide for the Selection of Filters from Spasciani, lists the odour threashold for many among more common gases. Some other gases, such as CO (Carbon Monoxide) have no smell and cannot be detected by odour. More information in the Informative sheet that come with every pack of product.
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