It really Helps us to help you, if you mention what program you’re using,
Carrara has Particle emitters which can be adjusted using the particle editor, and controlled in the scene through the time-line sequencer, and by physical forces in the scene.
Bryce has a Particle emitter, but there is less ability to control the particle shape, timing, and motion of the particles,
Poser has a Particle effect plugin product.
Daz Studio has a “Particle FX” product
As you mention the Particle Emitter “Room” then I’ll assume you’re using Carrara.
There are options to control the “Age” of the particle emitter, in the bottom left hand section of the particle editor,.
you can set the overall Life of the particle, and in the central section, you can set the “From” and “To” time, in seconds.
normally this is set to start emitting particles at 00:00:00 , until ten seconds 00:10:00
If you wanted an emitter to start emitting particles at two seconds, you would change the “From” time to 00:02:00
There is also an option to Pre- generate particles before the start of your animation, and you can add a time value here to specify how long before the start of your animation particles should be generated.
For example a moving “steam train” with a smoke trail may look strange if it only started emitting smoke only when your animation started .
Hope it helps