Flora: In general, the endemic and island-adapted flora of Hawai’i are very benign. Things like poison ivy are rare if nonexistent. Kiawe Trees have really nasty thorns that can penetrate hard rubber soles. Don’t underestimate them, especially when they are buried in the sand.

Fauna: The biggest wild animal you will encounter are the feral pigs that roam the more natural parts of the island. Ruinous to the endemic rainforest, you can tell you’re in a pig area by the obvious damage done to the tree ferns. Encounters with humans are rare and a feral pig will probably run away from you, but they can be quite large (up to 200 pounds) and deserve your respect. There are a few species of scary-looking spiders like the cane spider, but none are poisonous (although a few are venomous and can leave a painful bite). There are no snakes (or shouldn’t be). The worst thing you can encounter is the notorious Hawaiian centipede. They can be black, bluish, or red, and can grow to six or so inches long. The bite is said to be intensely painful (but not life threatening). There is no antidote, and a local legend says to just stay drunk for a few days. Centipedes thrive in wet, warm places. Mosquitoes congregate in certain locations on the island, and their possible presence is noted on several hikes on this site. As a person who is especially attractive to mosquitoes for whatever reason, I have only gotten a few bites in obvious places and rarely feel the need to apply a DEET based repellent. Just a light breeze can almost completely get rid of mosquitoes immediately.

Hypothermia: While hypothermia is likely less of a concern along the coasts, it is an ever-present concern in the higher country of the Big Island. Hypothermia results when your body’s internal temperature drops to a point (about 95 degrees) where your brain reacts by beginning to shut down non-essential systems. Common symptoms of mild hypothermia are confusion and severe shivering due to chills. Serious hypothermia can result in organ failure and death. The key to avoiding hypothermia is proper clothing and gear. Always carry raingear in the rainy, cool parts of Hawai’i like the National Park. Cotton clothing exacerbates heat loss. Polyesters and wool, on the other hand, retain warmth when wet.

Heat Injuries: While hypothermia occurs when body temperature drops, hyperthermia occurs when the opposite occurs. Heat injuries are a big concern when hiking in any coastal region. Mild symptoms can include reddish skin, confusion, vomiting, diarrhea, nausea, and clammy skin. Heat injuries typically occur as a result of overexertion in the outdoor hiking context. Avoiding heat injuries is fairly simple: hike at a moderate pace; take scheduled breaks in the shade; drink and carry plenty of water; consider covering bare skin with light-colored long pants and shirts made of polyester; wear a hat that shades the sun; wear sunscreen; and above all know your limits.

Dehydration: The tropical sun in Hawai’i is unrelenting. Despite the humidity in the air, you’ll need to drink and carry water as if you were in a desert. Dehydration can occur faster than you think, so drink water before you’re thirsty. If you’re partying at night, note that alcohol can exacerbate dehydration.

Acute Mountain Sickness (AMS): This only applies if you are going to visit Mauna Kea or Mauna Loa. This serious medical condition is a result of a lack of oxygen in the atmosphere at higher altitude, so if you don’t intend to visit the high volcanoes of the island, don’t worry about this. If you do intend to climb above 8000' (especially if you intend to drive up to the top of Mauna Kea) read this extended discussion of Acute Mountain Sickness

Water (freshwater rivers and streams): Water crossings of any kind should be carefully evaluated on the Big Island. This is constantly-changing land. Swift rivers and streams kill people every year in Hawai’i. Notably, the Wailuku River near Hilo claims lives every year. Maybe if they translated it, it might help: it means river of destruction. Always consider flash flood danger and understand that it doesn't matter if it's raining where you are -- it only matters if it's raining upstream. There are a few hikes on this site that require you to cross flowing freshwater. When in doubt, use a rope -- or turn around and come back another day. 

Water (drinking): Treat all sources of freshwater on the Big Island before drinking. There are no known freshwater sources that are safe. Giardia and Cryptosporidium are common, as well as some other less common parasites. Beyond parasites that you ingest via drinking water, Leptospirosis spirochetes can enter the body via soft tissue, such as open cuts or sores, the nose, ears, and mouth. Avoid swimming in freshwater if you have open cuts or sores, and consider keeping your head above water in areas known to contain Leptospirosis, like Waipi'o Valley. This disease usually doesn't show symptoms until two weeks after expsoure and can be fatal. If you feel sick with flu-like symptoms a few weeks after hiking or swimming in freshwater sources, seek medical attention immediately and inform your healthcare provider that you might have been exposed to Big Island freshwater parasites. 

Water (ocean): If the rivers and streams weren’t dangerous enough, the ocean around Hawai’i  is extremely dangerous and claims many lives every year. None of the hikes on this site require you to swim in the ocean, but several lead you to beautiful beaches. This beauty usually comes along with strong rip currents and submerged, dangerous lava. Never turn your back to the ocean. If you get caught in a rip current, calm down, let it drag you out, and then swim parallel to the beach to escape its grasp. Don’t assume that it’s possible to swim at every beach, no matter what you’ve heard or read. Carefully research and evaluate the options. Talk to locals. A few hours can drastically change beach conditions. Sharks, as usual, are an overblown but present threat in the waters. Avoid them by staying out of murky water, especially at dawn and dusk. Sea urchins are present in many offshore areas, and you really don’t want to step on one. Water shoeshelp a lot to avoid them, and the sharp lava too. A more pressing concern in the ocean is Portuguese man-of-war jellyfish that probably won’t kill you but will certainly ruin your day. If you get stung, first remove any remaining tentacles from your skin with a stick or gloved hand with freshwater or seawater to rinse. Don’t put anything else on the wound. None of the absurd old wives' tales work – not vinegar, not urine(!), nor any of the things they sell in stores. These methods are either scientifically inconclusive or can even make it worse (especially urine – please don’t piss on your hiking partner, literally or figuratively). Apply hot or cold depending on which feels better to the victim. Contact emergency medical services if cardiac issues or severe confusion develops in the victim.

Air Quality: There is a lot of sulfur dioxide gas issuing from Kīlauea volcano, and the strong trade winds usually carry it across the island. The term for the pollutant that results is vog, a combination of “volcanic” and “smog.” It occurs from the chemical reaction of sulfur dioxide and other gases from erupting volcanoes mixing with oxygen and water vapor in the presence of sunlight. It’s usually no worse than a bit hazy on the Kailua-Kona side, but gas can be strong in the Kaʻū region of the island. As you get closer to and inside Hawai’i Volcanoes National Park, certain areas downwind of both Halema’uma’u Crater and Puʻu ʻŌʻō can be very bad. Sulfur Dioxide gas is a severe irritant to the respiratory system (trust me), and is especially dangerous to pregnant women, the elderly, and those with respiratory problems. I always carry an inexpensive painters’ mask with me when I hike in the National Park and Kaʻū District. It weighs almost nothing and has saved me on several occasions. There is more on this in the Hawai’i Volcanoes National Park section of the site.

Lava Tubes: A lava tube is essentially a lava cave: these tubes carry magma from eruptions, usually out to the sea. As the lava cools, the walls of the lava tube form. Once formed, they often see several lava flows at different times. Lava tubes are literally everywhere under your feet on the Big Island. A cross-section of the island would look like swiss cheese. Exploring lava tubes usually involves finding a “skylight,” which is nothing more than a collapsed roof that often provides an easy climb down into the tube over rubble. Then, most accessible lava tubes just involve some difficult and dark hiking. Lava tubes are generally safe when old and cool and usually collapse during the cooling process once lava stops flowing. Usually. Exploring a lava tube is much more akin to hiking than caving in this respect. That said, exploring lava tubes deep beneath the earth carries a set of risks. Probably the biggest hazard is you: you’re more likely to run out of light, get lost, or get hurt than have a cave-in or some other natural hazard. When we hike lava tubes, we always wear a helmet. I like to use a rock climbing helmet because they are lightweight, comfortable to wear for a long time, usually allow for easy attachment of a headlamp, and protect best against falling rock. I wear a powerful headlamp on the helmet, and carry another flashlight in my hands. Always carry extra batteries. If you got caught in a lava tube without light, it would be nearly impossible to get out safely. No one will hear you scream. No matter what the weather is like above ground, lava tubes generally hold a nice temperature in the upper sixties and are quite humid. Wear long pants and carry a light jacket or sweater. Leather gloves can be handy both to keep your hands warm and to protect them against jagged lava. In any case, always wear gloves so you don’t leave skin oils that might disturb cave-adapted species. If possible, try to touch nothing in the tube with your bare skin. Lava tubes were used by ancient Hawaiians to inter their dead and also for certain spiritual practices, so if you happen upon any archaeological remains in a lava tube, touch nothing and leave no trace. Consider reporting your find to the University. If you’d like to explore lava tubes in a low-risk environment, try Nāhuku (Thurston Lava Tube) in Hawaii Volcanoes National Park. This small tube has a paved walkway and lighting. 

Tsunami: It’s not a major concern, but is worth mentioning. When you’re in any coastal region of the Big Island, you’re in the tsunami danger zone. Almost every state and county road contains a sign at about the 500’ level indicating safety from these killer waves. If you hear sirens, head quickly for land above these signs. The danger is more notable when hiking the southern coastal region of Hawai’i Volcanoes National Park where earthquakes are more common. If you feel the ground shake here, hike to higher ground immediately.

Theft at Trailheads: This isn’t a major problem, but does occur at some popular trailheads (this isn’t a concern, say, at the Mauna Loa trailhead at 11,000 feet). Locals, mostly teens, sometimes case popular parking lots at trailheads to engage in petty theft. Rental cars are obvious, as are tourists. It’s a crime of opportunity: wait for tourists to load up a backpack and head off down the trail, and then break into their car once they are gone. The best way to avoid theft is to leave nothing of value in your car and leave the doors unlocked. Don’t put anything in the trunk. My strategy is to look like I own nothing of value by my generally unkempt appearance and odd demeanor.