It'll pass up the stairs

Emergencies
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Reporter: Now that we know how to cook more safely in the kitchen, we're going to talk about smoke detectors in the rest of the house.

Captain: A smoke detector, mostly called a smoke alarm,
is a must for fire safety in a home. You should have at least one
and it should be placed on a ceiling near where you sleep,
just outside your bedroom. We have one located on the ceiling here right by the stairs, so if there's smoke in the living room, it'll pass up the stairs and go by the alarm and cause the alarm to go off.

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it'll pass up the stairs and ...

I don't understand.

Which is correct:

1- it'll go up the stairs ? Vertical movement of smoke

2- it'll pass over the stairs ? horizontal movement of smoke

Is it a horizontal movement of smoke or Vertical movement of smoke?

Thank you