Steam and gas rise from the Kilauea volcano’s summit crater Tuesday in Volcanoes National Park, Hawaii. Geologists warned Wednesday that Kilauea could erupt explosively and send boulders, rocks and ash into the air around its summit in the coming weeks. (Jae C. Hong/AP)

Among the many threats Hawaii’s Kilauea volcano poses — dangerous sulfur dioxide concentration, lava flowing through neighborhoods, boulders-turned-ballistic missiles — is acid rain, says the U.S. Geological Survey. But it’s not as bad as it sounds.

It’s not going to “rain acid” on the Big Island. There wouldn’t be enough of it to do much more environmental harm than what occurs there on a daily basis (Kilauea has been erupting since 1983, after all). And it’s certainly not going to melt plants and burn your skin, says Daniel Jacob, professor of atmospheric chemistry at Harvard University.

“Sulfur dioxide in surface air is not good for vegetation,” Jacob said, “but it’s not that soluble in water.” Meaning, acid rain isn’t going to form when rain falls through sulfur dioxide — it’s a much longer process than that.

Even if some acid rain did form, Jacob said, it’s not something necessarily to be alarmed about.

“It’s not like you’re going to walk outside, and your skin is going to itch and your eyes are going to burn,” Jacob told The Post.

The pH scale measures how acidic or basic things are. Pure water is completely neutral at a pH of seven. Anything less than seven is considered acidic, and anything higher is alkaline (basic). The scale is also logarithmic, which means, for example, that a pH of five is 10 times more acidic than a pH of six and 100 times (10 x 10) more acidic than a pH of seven.

Acid rain has a pH around 4, while battery acid is as far down on the scale as it can get — 0. Applying our new knowledge of the logarithmic scale, we know that battery acid is 10 x 10 x 10 x 10 = 10,000 times more acidic than acid rain. Even lemon juice and orange juice are more acidic than acid rain.

Acid rain, however, is harmful to plants and animals, and over a long period it can damage infrastructure (imagine something along the lines of skyscraper ceviche). The environmental effects are really pronounced in aquatic ecosystems, where acid rain can leach aluminum out of the soil as deposit it in streams, rivers and lakes, according to the Environmental Protection Agency.

Acid rain aside, there are much bigger risks on the Big Island right now, says Jacob, including the very high concentration of sulfur dioxide gas in the air.

“SO2 exposure is not good for vegetation or people,” he said. “There’s a reason it’s regulated by the EPA.”

A visitor walks along a trail as steam and gas from Kilauea fill the air in Volcanoes National Park. (Jae C. Hong/AP)

The chemical compound damages the human respiratory system and makes breathing difficult, according to the EPA. The effect is most pronounced in kids and the elderly, but if sulfur dioxide concentration is high enough, even well-trained athletes can be harmed.

Vog, or volcanic fog, is a “hazy mixture” of sulfur dioxide and other tiny particles and compounds, and it’s not good to breathe. For vog to form, sulfur dioxide gas needs to be festering in sunlight with oxygen, moisture and these other compounds. According to the U.S. Geological Survey, “the exact composition of vog depends on how much time the volcanic plume has had to react in the atmosphere.”

Typically, prevailing winds off the ocean will disperse the chemicals. On Thursday and Friday, winds will be relatively calm, which increases the likelihood of vog. The National Weather Service says vog could become widespread on the Big Island Thursday afternoon through Friday night. If it does, officials probably will recommend that people stay inside until winds shift.