This experiment could use an additional negative control: Fan with no filter. There are many possible explanations for the observed data, for example, the air

This experiment could use an additional negative control: Fan with no filter. There are many possible explanations for the observed data, for example, the air movement through the fan changes the static charge of the smoke particles, causing them to cling to objects in the room. Or, the smoke particles could be binding the plastic blades of the fan. Or, distributing the smoke throughout the room dilutes the smoke concentration at the sensor. For fun, ask your TA for the full list of possible hypotheses! At this point, we can't say that the HEPA filters are causal. The tape versus no tape experiment produced unexplained and surprising results, which is even more reason to run careful negative controls. Now, if we could easily label the smoke particles, e.g., using a fluorophore, and measure their distribution after filtering, we'd know exactly where they're going.