Vog is volcanic smog: sulfur dioxide from the vents reacting in the atmosphere to a haze of fine sulfate particles. HVO puts summit emissions during pauses at 1,000 to 5,000 tonnes of SO₂ per day; during a fountaining episode they are far higher. Where it goes depends on the wind. With trade winds from the east – the forecast for October 2 – the gas is pushed over the closed southwest part of the caldera and toward the Kaʻū and Kona side of the island; when the wind drops or turns, it can sit in the summit area.
The NPS counts infants, children, pregnant women and anyone with heart or respiratory conditions among the sensitive groups and asks all visitors to check air quality before and during the visit, and to leave an area when you smell sulfur or feel your eyes and throat sting. Two bookmarks do the job: the vog dashboard of the International Volcanic Health Hazard Network, which combines forecasts with the Hawaiʻi Department of Health's monitors, and the live air-quality map of the Hawaiʻi Department of Health. On episode days add tephra and Pele's hair – strands of volcanic glass carried downwind and remobilized for days: long sleeves, eye protection, a dust mask, and do not touch the strands.