A fever becomes dangerous based on both how high the temperature is and how the person looks or feels, but there are some clear “red line” numbers to know.

Key danger temperatures

For adults:

  • 100.4°F (38°C) and above is considered a fever.
  • Around 103°F (39.4–39.5°C) is usually considered a high fever and you should contact a healthcare provider, especially if it lasts or you feel very unwell.
  • 104°F (40°C) and above is often treated as an urgent situation; many sources recommend being seen right away.
  • 105–105.8°F (40.5–41°C) and higher is dangerous and can start to damage organs and the brain if sustained; this is an emergency level and needs immediate care.

For babies and children:

  • Any fever in a baby under 3 months old of 100.4°F (38°C) or higher needs urgent medical evaluation, even if they don’t look very sick.
  • For older infants and children, 102–103°F (38.9–39.4°C) that doesn’t improve with medicine, or keeps coming back, deserves prompt medical advice, especially with other worrying symptoms (rash, vomiting, trouble breathing, unusual sleepiness).
  • Fevers at or above 104°F (40°C) in children over 3 months should be evaluated quickly by a doctor or urgent care.

When it’s truly dangerous vs just “high”

A high number alone is worrying, but “dangerous” usually means:

  • Very high temperature (around 104°F/40°C or above).
  • Or any fever plus serious symptoms like:
    • Trouble breathing or chest pain.
    • Confusion, acting very strange, or not waking normally.
    • Stiff neck and bad headache.
    • Seizure.
    • Signs of dehydration (very dry mouth, not peeing, no tears).

Even moderate fevers can be more dangerous in people with heart disease, lung disease, weak immune systems, or dementia, because fever stresses the heart, lungs, and brain.

Simple example

Imagine two adults:

  • Person A: 102.5°F, drinking fluids, alert, mild aches, otherwise okay → often safe to monitor at home with rest, fluids, and fever reducers, unless they’re high risk or it lasts several days.
  • Person B: 101.5°F but very short of breath and confused → much more dangerous and needs emergency care, even though the number is lower.

When to get help right now

Get emergency help (ER or emergency number) if:

  • Temperature is about 104°F (40°C) or higher in an adult or older child, or
  • Any baby under 3 months has 100.4°F (38°C) or more, or
  • There is confusion, trouble breathing, chest pain, blue lips/face, seizure, stiff neck, or they are very hard to wake.

If you or someone near you has a fever close to or above these danger levels, or looks very sick, treat this as urgent and seek medical care immediately. This information is general and does not replace in‑person medical advice.