Air filter should be changed at least once a year or whenever the indicator goes off on the dashboard, whichever comes first.
The filters in an air purifier ususally need to be changed around every 3 - 6 months but it also depends greatly on what kind of air purifier you have .
All of them
Some air purifiers do require a filter to be changed others have a filter that can be cleaned.
The filters are what needs to be replaced most often on the air conditioning units.
The filters in an Ionic Breeze dont not require a filter change.
How often air filters must be changed depends on your local air quality: the dirtier your air is the more frequently you should change your air filters. In an average city environment about once every 3 months may be ok but if your air is very polluted then every month may be necessary. In very clean air you may only have to change them once a year.
Air conditioning filters have to be cleaned at least every three months. Dirty air conditioning filters can increase energy consumption of air conditioners.
They get dirty and restrict the air flow.
The engine oil should be changed at least every 3,000 miles on a 1985 Toyota Tercel. An automobile this old should also have the oil and air filters changed at the same time for the motor to run move efficiently.
2004 Sequoias do not have in cabin air filters.
It is recommended that air filters for cars and trucks should be changed every 15,000 miles or once a year. This standard is for city vehicles and would be different for those who drive in rural areas. Air filters in rural areas are subject to more dirt accumulation and may need more frequent replacement.
I would look into the Hepa Filters, I ask your allegist if they have a recommediation. I have had real good luck with the hepa filters as long as you keep filters clean and changed.