Method of estimating filter lifetime

A method of estimating S130 a lifetime of a particulate filter (120, fig. 2) installed in an apparatus (100, fig. 2) using retrieved datasets including: a first dataset S110 indicative of performance of a motor for generating an airflow through the filter over a time period since installation of the filter; and, a second dataset S120 indicative of an amount of particulates in the airflow over the same time period. An indication may be provided to a user when the estimated lifetime of the filter has reached a design lifetime. Other datasets may also be retrieved to make the estimation relating: to temperature, barometric pressure and humidity. The second dataset may be generated using a sensor (160, fig. 2) configured to detect particles of 4.5 microns or smaller in diameter. An air purification system and method of training a machine learning module have also been claimed.