If you're flying to the Big Island this summer, there's one local weather pattern that rarely shows up on vacation checklists until it's already in your eyes: vog. It's short for volcanic smog, the haze Kīlauea sends downwind whenever the volcano is venting sulfur dioxide. Which is almost always.

The reason to think about it now: Kīlauea's summit eruption is in a pause between fountaining episodes. The USGS Hawaiian Volcano Observatory reports that episode 44 ended April 9, and forecast models place episode 45 between April 20 and April 25. The alert level sits at ADVISORY and the aviation color code is YELLOW. None of that keeps you off the island. All of it changes what a clear day looks like.

Here's what summer travelers should know before the trip, especially anyone with asthma, kids, or older parents along.

Vog is mostly sulfur dioxide gas (SO2) plus a fine sulfate aerosol that forms when SO2 reacts with sunlight, oxygen, and moisture in the air. You see it as a grayish-white haze that can flatten views, soften sunsets, and settle into the leeward valleys on light-wind days.

Even while the summit eruption is paused, Kīlauea is releasing 1,000 to 5,000 tonnes of SO2 per day, per the USGS daily update. That's plenty to affect air quality across the island, depending on wind.

When northeasterly trade winds are blowing, which is most days in summer, vog pushes southwest across the island and piles up on the leeward Kona coast. Kailua-Kona, Kealakekua, Captain Cook, and the resort strip from Keauhou down through South Kona can see hazy skies even when the summit area is crystal clear.

When trades slack off or wind shifts to the south (a "kona wind"), the pattern flips. Vog blankets the summit region, Volcano Village, Kaʻū, and parts of the east side including Hilo. The National Park Service has flagged exactly this risk for the next eruptive episode, warning that southerly winds could blanket the summit with gas and fine volcanic glass (Pele's hair).

The practical version: "vog affects the Big Island" isn't quite right. Vog affects whichever side of the island is downwind of Kīlauea that day. A quick reference:

• Trade-wind days (typical summer pattern): Kona side gets the haze. Hilo and the windward side usually stay clearer.
• Light-wind or south-wind days: Volcano Village, the national park summit area, Kaʻū, Nāʻālehu, and sometimes Hilo bear the brunt. Kona may clear up.
• Kohala and high elevations: Waimea and the upper slopes of Mauna Kea often sit above or upwind of the densest vog layer, though conditions vary day to day.

Which means flexible itineraries beat rigid ones on the Big Island. Don't assume Hilo is the automatic escape from a hazy Kona morning. Check an AQI map first (sources below), and let the wind that day decide which side of the island you spend it on.

The Hawaiʻi Department of Health flags several groups as most sensitive to vog: people with asthma, bronchitis, emphysema, or other chronic lung disease; people with heart disease; children, whose lungs are still developing; older adults; and pregnant travelers.

Short exposures to moderate vog typically cause eye and throat irritation, a scratchy cough, or extra mucus. Longer exposure or higher concentrations can trigger asthma attacks, chest tightness, or shortness of breath. If you have asthma, bring your rescue inhaler and your daily controller. Not just one or the other.

Three sources are worth bookmarking for this trip:

• Hawaiʻi Interagency Vog Information Dashboard: the consolidated hub run by the International Volcanic Health Hazard Network with state agencies. It links out to forecast models (VMAP), real-time SO2 and particle data, and visitor-specific guidance.
• AirNow's Hawaiʻi page: color-coded Air Quality Index readings from the EPA network. Green is fine. Yellow is "moderate" and usually fine for most people. Orange, red, and purple are when sensitive groups should start scaling back outdoor plans.
• Hawaiʻi Volcanoes National Park air quality alert page: the park's own posted conditions and any area closures tied to gas levels.

Phone weather apps will sometimes show an AQI value, but accuracy and update frequency vary by app and location. For a real read on a haze morning, go straight to AirNow or the IVHHN dashboard.

The Department of Health's standard guidance on a high-vog day: cut back on strenuous outdoor activity, stay inside with the windows closed, set air conditioning to recirculate rather than pulling in outside air, keep rescue inhalers and respiratory medications close by, and drink extra water. Save the hike or the 5 AM snorkel for a clearer day.

One thing DOH specifically does not recommend: wearing a dust mask or N95 against SO2. The gas molecules pass right through them, and wearing a mask can give a false sense of protection. If vog is bothering you, the answer is getting indoors with conditioned air. Not a mask.

The overlook areas near Halemaʻumaʻu, including Keanakākoʻi, Kīlauea Overlook, and parts of Crater Rim Drive, are where vog exposure is most direct and most variable hour to hour. The Park Service routinely closes specific overlooks on heavy gas days and reopens them when the air improves.

If you're heading in for an eruption episode, check Alerts & Conditions and the air quality alert page before you drive up, and again at the visitor center when you arrive. If a particular overlook is closed, another is usually open. Park staff are the most reliable source of day-of guidance, and they answer the phone (808-985-6011).

Pele's hair, the thin strands of volcanic glass carried on the wind during fountaining, is another thing to know about. Don't touch it. It breaks into tiny pieces that can embed in skin and eyes.

A car rental matters more on the Big Island than on any other Hawaiian island, and vog is one of the reasons. The island is big enough that a 45-minute drive can take you from hazy skies to clear ones. If you're staying in Kona on a hazy morning, you want the option to drive east toward Hilo or up to Waimea (assuming the wind that day actually has those areas in clearer air). A rental keeps that option open.

If you'd rather not drive the Saddle Road yourself, day tours from Kona and Hilo cover Hawaiʻi Volcanoes National Park. If air quality is a concern, ask the operator before booking what they do when the summit area is closed for gas levels.

Vog isn't a reason to skip the Big Island this summer. It's a reason to travel with a little more flexibility than you would on Maui or Kauaʻi. Check the air in the morning, know which side of the island is downwind that day, keep your medications accessible if you have asthma, and don't take a Halemaʻumaʻu selfie if the overlook is closed. It's closed for a reason.

Episode 45 is expected to begin between April 20 and 25, and once it does, fountaining episodes through summer are possible if the pattern that's held since December 2024 continues. A lava fountain is one of the most extraordinary things you can see as a visitor to Hawaiʻi. It also comes with more gas, more Pele's hair, and more day-to-day variation in where you can comfortably stand and breathe. Both are true at once.

If you're planning a summer trip now, the boring advice is the useful advice: build in a buffer day, don't book everything on the same side of the island, and bookmark the IVHHN dashboard so you can glance at it the way you'd glance at a surf forecast. The Maunakea Visitor Information Station page (run by the Center for Maunakea Stewardship) is worth a look if you're hoping to escape vog at altitude on a stargazing night.