Air filtration at elevated temperatures presents multiple challenges in filter performance and filter integrity testing. Many standard cleanroom practices and HEPA filter test procedures do not work, and some may result in hazardous operation of equipment.

Most current high efficiency air particulate (HEPA) and ultra low particulate air (ULPA) filters are designed and constructed to accommodate ambient and near ambient temperature conditions. They can achieve levels of removal of 0.3-micron and larger particles that exceed 99.95%, and, depending on the concentration of particles in the incoming air, produce air of Class 1 (ISO Class 3), Class 10 (ISO Class 4), or Class 100 (ISO Class 5) which, in English units, are particles of 0.5 micron and larger particles per cubic foot.

However, when these filters are used to perform the same filtration function at temperatures exceeding 180°C, many factors, including temperature ramp rate, method of challenge for integrity screening, and seals need to be modified to achieve the best and safest end result. Failure to recognize these differences when utilizing filter techniques at elevated temperatures may result in, at a minimum, substandard filter performance. At worst, high temperature operation may present physical danger to the operators and processes being performed in the equipment.

The primary application of HEPA and ULPA filters at elevated temperature equipment occurs in ovens designed for use in the medical device, pharmaceutical, and microelectronics and semiconductor manufacturing operations. These ovens may be performing sterilization or depyrogenation of instruments or glassware for use in the life sciences or be used for holding, curing, and/or annealing electronic components that must be kept clean. These processes can range in temperature from 100 to 400°C and require temperature ramping rates from steady state (i.e., parts introduced at the process temperature) to as much as 15+ C°/min. Hence, high temperature use of HEPA filters can present a number of challenges to the use of filters which we most commonly associate with ambient operation in a cleanroom.