From Brotato Wiki
Each level inherits the difficulty increases from level before them.
Danger 0
No Modifiers
Danger 1
New Enemies Appear
Danger 2
Inflation is increased by 5% (needs confirmation). After each wave item prices in shop increased by total inflation. After first wave item prices are increased by 5%. After second wave item prices are increased by 10%. On the last wave item prices are increased by 100%.
Danger 3
Enemies are stronger. Enemies hit 5% more, have 5% more hp, and run 5% faster.
Danger 4
Enemies do not drop materials 5(needs confirmation)% of the time.
Danger 5
On the 20th wave of the run, two bosses spawn instead of one.