If a person holds his breath for 25 seconds after expiration, even then exchange of respiratory gases would occur in lungs. It is so because, even after we exhale, some amount of air remains in the lungs. This volume of air is called as residual volume which has a quantity of about 1200ml. So even if a person hasn't inhale the air for 25 seconds, then also this residual volume will continue gaseous exchange in lungs for some time. But if this period last some more longer, then this can be lethal.