I've found a smoke test to be a poor indicator of the quality of the ventilation because smoke from say a burning piece of paper or incense is very lazy, any amount of air flow will capture it and move it along.
If however, the smoke has velocity such that it can escape the suction of the fan it's going to collect in the head space of the room. When that happens it's like filling a pool upside down where the fumes gather and move downward. They will eventually reach the exhaust, but if it's lower than head height you'll be inhaling them at the same time.
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