Three symbols in "To Build a Fire" that reinforce the theme of isolation are the frozen terrain, the lack of sun, and the old man of Sulphur Creek.
The cold and frozen terrain symbolizes death and isolation. As the man sets out on his journey, he surveys the landscape:
The Yukon lay a mile wide and hidden under three feet of ice. On top of this ice were as many feet of snow. It was all pure white. North and south, as far as his eye could see, it was unbroken white.
This vast, empty, and frozen terrain presents a physical challenge to the man, yet he fails to consider how his own "weaknesses as a creature" might be revealed in temperatures that are fifty degrees below zero. This frozen landscape presents a physical danger, isolating the man from any real source of intervention. He walks along the frozen stream where "no man had traveled" in over a month.
It is also worth noting that the lack of sun is repeated numerous times in the story:
There was no sun or promise of sun, although there was not a cloud in the sky. It was a clear day. However, there seemed to be an indescribable darkness over the face of things.
The sun often symbolizes life and energy, so the lack of sun as the man begins his journey therefore symbolizes destruction and weakness.
The old man on Sulphur Creek symbolizes a healthy respect for the dangers of the natural world, which the man ignores. The old man man cautioned that "no man should travel alone" in temperatures that reach fifty degrees below zero, but the man believes that he is fully capable of "sav[ing] himself." It isn't until the snow falls on his fire that the man recognizes the danger of isolation; he ultimately recognizes the danger of isolating himself in a frozen world only when it is too late.