There are actually 5 intake holes, which can be blocked if needed. Every ventilation system needs exhaust (in my case HEPA+CARBON filter) and INTAKE (in my case a bit of foam, to prevent sucking in the dust from workshop). INTAKE needs to be 2x larger than exhaust in surface area. Larger the intake, less the air disturbances and air speed. When you have exhaust, you need to have intake and air WON'T leak anywhere, because whole chamber is at slight underpressure. Only air leaving the enclosure will be through HEPA/CARBON filter. :) If you got a lot of leak holes in the enclosure and nothing sealed etc., you might get away without intake holes, but fan will probably be struggling.