There is a greater risk of explosion with dust in the air rather than dust on ground. This due to the fact that dust combusts quickly and air supports combustion while the ground does not.

Cause dust can combust real fast and air supports burning the ground doesn't

All we do crumbles to the ground though we refuse to see. Everything is dust in the wind.

They pick it up from the ground

It means we were made from the dust and when we die we will return to the dust because our bodies disintegrate and we return to the ground.