The use of portable air cleaners is becoming increasingly popular in many countries including Denmark. Portable air cleaners are known for not only removing but also generating particles and gases. To clarify this, three air cleaning technologies were evaluated. They were non-thermal plasma, photochemical air purifier and corona discharge ionizer. The concentrations of ultrafine particles, ozone and total volatile organic compounds were measured both in a duct and in a clean room. It was found that the studied air cleaning technologies increased the ozone level in the clean room and the duct. The increase of ozone level in the clean room was more than that was measured in the duct. Additionally, it was found that the number of ultrafine particles in the room increased due to the generated ozone. The number of generated particles changed with the season. The study leads to the recommendation that air cleaners should be evaluated in a clean room about generation of ozone to get more reliable evaluation.