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Trail terms

Air-activated warmer

Also called hand warmer, toe warmer, heat pack

What is an air-activated hand warmer?

An air-activated warmer is a sealed, single-use pouch of iron powder, salt, water, charcoal and vermiculite that heats up when its wrapper is opened. The iron rusts in the air, giving off steady heat for several hours, which is why it needs air to keep working.

Air-activated warmer at a glance

What reacts
Iron powder with oxygen
Typical duration
About 8 to 18 hours by size
Reusable
No, single use
Placement
Over fabric, never bare skin
Toe warmers stick to the outside of a sock

How does an air-activated warmer make heat?

It rusts on purpose. Opening the wrapper lets oxygen reach fine iron powder, and the iron oxidizes, releasing heat as it does. Salt speeds the reaction up, vermiculite holds the water it needs, and charcoal is part of the mix, per HotHands' ingredient list.

The reaction keeps going until the iron is used up, so a warmer is single use. Shaking it after opening helps air reach the powder and brings it up to temperature faster.

How long does a warmer stay warm?

Makers rate their packs by size. HotHands, for example, rates its standard hand warmers for up to 10 hours, its thinner toe warmers for up to 8 and its larger body warmers for up to 18. Those are best-case figures with plenty of air.

A warmer starved of oxygen runs cooler and slower. That is why toe warmers are made thin and adhesive, to sit on top of a sock rather than under the foot, and why one sealed inside a tightly laced boot can feel weak.

Is it safe against skin?

Keep it off bare skin. HotHands says its warmers run anywhere from about 100 to 180 degrees Fahrenheit depending on the product, and long contact at those temperatures can cause a burn, especially on numb skin or for someone with poor circulation. Put it in a glove, a pocket or over a layer of fabric.

What this means when you're choosing

  • A stated duration on the pack, since sizes differ a lot
  • An adhesive back on toe warmers, so they stay put in a boot
  • A larger body warmer for a full winter day or an overnight
  • Sealed individual wrappers, since an opened pack starts heating

The common mistake

Sealing it where it cannot breathe

A warmer packed into a tight boot or a sealed mitt with no air around it slows right down, and people blame the warmer. Leave a little room in the toe box, and open the pack a few minutes before you need the heat.

Common questions

Can you reuse a hand warmer?

No. The iron inside is used up as it reacts. Sealing a half-used warmer in an airtight bag slows the reaction and saves some of the heat for later, but how much comes back varies.

Why is my toe warmer not getting warm?

Usually because it is not getting enough air. A tightly laced boot cuts off oxygen, so the reaction slows. Loosen the toe box a little, and give the warmer a few minutes in the open before putting the boot on.

Where this is defined

Where this comes up

Guides on Tom's Hiking where air-activated warmer changes which product is right for you.

Going deeper

More in trail terms

Base weight

Base weight is everything in your pack except food, water and fuel: the gear that doesn't get used up. It's the number backpackers compare, because consumables vary with trip length while base weight is a property of your kit.

Ultralight

Ultralight is a style of backpacking built around a base weight under 10 lb. It gets there by making the big three lighter and by carrying fewer things, which trades comfort in camp and margin in bad conditions for comfort while walking.

Fastpacking

Fastpacking is covering a multi-day route with a pack light enough to run in, somewhere between trail running and ultralight backpacking. Typical loads run 8 to 20 liters, and the trip is usually run downhill and on the flat and hiked uphill.

Thru-hiking

Thru-hiking is walking a long trail end to end in one continuous trip, usually within a single season. The Appalachian Trail, Pacific Crest Trail and Continental Divide Trail are the best-known examples in the US, each taking several months.

Shoulder season

The shoulder seasons are spring and autumn, the months either side of settled summer conditions. Days can be warm and nights near freezing, weather changes faster, and gear that works in July is often not enough in late September.

Three-season

Three-season gear is built for spring, summer and autumn but not sustained winter conditions. It's the default rating for most backpacking equipment, and in practice it means handling rain, wind and light frost but not deep snow.

Layering system

A layering system is the practice of combining a moisture-moving base layer, an insulating mid layer and a protective shell, so you can regulate temperature by adding and removing pieces rather than relying on one heavy garment.

Zero day

A zero day is a day on a long trail where you walk no trail miles at all, usually taken in a town to rest, resupply and deal with logistics. A nero is a near-zero: a handful of miles into or out of town with the rest of the day off.

Nero day

A nero day, short for nearly zero, is a day on a long trail when you hike only a few miles and spend the rest of it resting or doing town chores. It sits between a full hiking day and a zero day, when you hike no miles at all.

Cowboy camping

Cowboy camping is sleeping outside with no shelter over you: a pad, a bag or quilt, and the sky. It saves the weight and the time of pitching a tent, and it trades away protection from rain, wind, dew, insects and anything that walks past in the night.

Postholing

Postholing is breaking through soft snow with each step and sinking to your shin, knee or thigh, leaving a line of holes behind you. It is exhausting, it can drop your pace to under a mile an hour, and it damages the trail for everyone following.

Slickrock

Slickrock is bare, exposed sandstone with no soil on it and no tread cut into it. You walk on the rock itself and follow cairns rather than a path. Dry sandstone gives excellent grip; wet or dusty sandstone gives very little, which is where the name misleads people.

Trailhead

A trailhead is the designated access point where a trail begins, and usually where you park and start walking. Many have a map board, signs or a toilet. The US Forest Service draws a firm line: a plain junction of two trails, or of a trail and a road, is not a trailhead.

Treeline

Treeline is the elevation, or latitude, above which trees can no longer grow. It is less a line than a zone where trees shrink into stunted, wind-shaped growth, and it sits at very different heights: between 5,000 and 6,000 feet in Washington's Olympics, and above 10,000 in the Sierra.

Backcountry

Backcountry is undeveloped land away from roads and drive-in campgrounds, reached on foot or by water. In US parks it is the opposite of frontcountry, where you drive to a campground, and camping there often needs a permit. It is not the same as legally designated Wilderness.

Big three

The big three are a backpacker's shelter, sleep system and pack: the three items where most of a kit's weight sits and, as REI puts it, where you can cut the most. The usual advice is to lighten the shelter and sleep system first and choose the pack last.

Section hiking

Section hiking is completing a long trail in pieces, one stretch at a time over separate trips, rather than in one continuous push. The Appalachian Trail Conservancy counts anyone who finishes the whole trail in more than 12 months as a section hiker, and recognizes them as 2,000-milers alongside thru-hikers.

NOBO and SOBO

NOBO and SOBO are thru-hiker shorthand for northbound and southbound, the direction someone walks a long trail. On the Appalachian Trail a NOBO starts at Springer Mountain in Georgia and finishes at Katahdin in Maine, and a SOBO walks the same 2,000-plus miles the other way.

Slackpacking

Slackpacking is hiking a stretch of a long trail with only a daypack while someone else moves the rest of your gear, usually a shuttle that drops you at one road crossing and collects you at the next. You still walk every mile; you just sleep indoors and carry less.

Naismith's rule

Naismith's rule estimates how long a hike will take: one hour for every three miles of distance, plus one hour for every 2,000 feet of climbing. A Scottish mountaineer wrote it in 1892 for fit walkers taking short halts, so most people need longer.

By Tom · Last reviewed . Back to the glossary