The initial concentration before the decay, is the maximum concentration Cmax. When t=τ (half-life or 1 air change), then the concentration in the room has reduced to 0.37Cmax. and when t=3∙τ, then the concentration C= 0.05.Cmax. This means that after 3 half-lives, the concentration in the room has been reduced by 95%. Therefore in practice, 3 τ is considered to be the time to achieve steady state conditions, for a rise, or the time to clear the room, for a decay.