I don't have a reference, but there are several factors involved:
1. The virus lives in the lungs, but the symptoms are experienced inside the body, mainly blood vessels.
2. Coughing obviously spreads more virus particles, but normal breathing does so as well.
3. The actual transmission mechanisms have not been well studied, both for practical and ethical reasons.
4. Spread is always a statistical matter, multiplying the duration of exposure, and the proximity of exposure.
5. Many households only have one person infected, not everyone is exposed.