When is a fever dangerous for my baby, toddler, or older child?

Multiple factors determine whether a fever may indicate a serious illness for your child:

  1. Behavior change
  2. Age
  3. Temperature
  4. Symptoms
  5. Fever duration

Among those variables, the degree of fever is the least critical. The best way to determine whether your child’s fever reflects a serious illness is through symptoms and behavior changes. If your child continues to eat, drink, and play normally, there’s less cause for concern.

However, significant behavior changes may indicate serious illness, even with a “low-grade” fever that falls below the medical threshold of 100.4° F (rectal). A temperature of 100.4° F is cause for concern only when your child is under 3 months of age. In infancy, a child’s immune system is unprepared to fight an infection, and you should see a physician if your young infant has a fever.

A healthcare provider should also be consulted if your child has a fever for more than three days, or if your child appears ill even when the fever is reduced with medication. Other reasons for seeking medical attention for fever include fussiness, lethargy, refusing to drink fluids, persistent vomiting, or difficulty breathing.

Fevers are not inherently dangerous and are self-limiting. Reaching a body temperature that causes brain injury (around 108° F) requires extreme environmental temperatures, like those of a closed car on a hot day. (Those dangerous temperatures have also been seen with rare adverse reactions to anesthesia.) For fevers caused by illness in children, the fever itself will never be high enough to cause brain injury, despite a common myth to the contrary.

Fevers by the numbers

While “normal” body temperature is 98.6° F (37° C), a child’s temperature can vary slightly from that standard. A normal temperature range is between 97.5° F (36.4° C) and 99.5° F (37.5° C). It also fluctuates throughout the day, with highest temperature between late afternoon and early evening, and lowest between midnight and the early morning.

Except for children under 3 months, deciding whether to visit the doctor should consider your child’s behavior as much as the thermometer’s reading. Even high fevers are common and not necessarily indicative of dangerous illness. For the most part, there is no direct correlation between the degree of fever and the severity of an illness.

What constitutes a fever also depends on how a child’s temperature is taken. Typically, rectal thermometers offer the most accurate temperature readings for infants and young toddlers; oral thermometers provide greatest accuracy for older children. (Most children can manage an oral thermometer at 4 years of age.)

Thermometer type Fever threshold
Axillary (armpit) 99.0° F (37.2° C)
Ear* 100.4° F (38° C)
Oral 100.0° F (37.8° C)
Pacifier* 100.0° F (37.8° C)
Rectal 100.4° F (38° C)
Temporal artery* 100.4° F (38° C)

*Ear, pacifier, and temporal artery thermometers are not as reliable as digital multiuse thermometers, in part because ambient temperatures affect their readings.

What is a fever?

A fever is the body’s natural response to illness—not an illness itself. For most children, a fever is a sign that their body is responding effectively to an infection. Fevers impede bacteria and viruses’ ability to replicate, and stimulate white blood cells to respond.

When viruses or bacteria invade your body, white blood cells release proteins called pyrogens. Pyrogens flow through the bloodstream to the hypothalamus, the region of the brain that controls body temperature. There, they bind to receptors that cause the hypothalamus to raise internal body temperature, resulting in a fever.

As the body’s immune response overwhelms the virus, white blood cells release fewer pyrogens, and body temperature gradually declines. Importantly, this process is self-limitingan immune response will not trigger a fever sufficient to cause brain injury. In short, a fever is protective and a good sign that the immune system is trying to fight the infection.

There is little to no scientific evidence to support the widely held belief that teething causes significant fever. Temperatures above 102° F (38.9° C) should never be attributed to teething alone.

What to do when your child has a fever

Caring for your child during a fever is less about managing the fever and more about your child’s comfort. If your child is comfortable, fever-reducing medications are unnecessary. While they temporarily reduce a fever, they do not enhance the body’s ability to defeat the underlying illness. Only when an infection has been eliminated will body temperature return to normal.

Acetaminophen (Tylenol) and ibuprofen (Advil, Motrin) are among the most common fever-reducing medicines. Dosing should be based on body weight. (KidMed maintains a free online dosing chart for acetaminophen and ibuprofen.) If your child is under 3 months of age, consult with your physician first before giving any fever-reducing medicines.  Avoid giving aspirin to any child under age 18. Aspirin can cause a dangerous condition known as Reye’s syndrome.

Other common at-home treatments, like cool baths or applying rubbing alcohol to the skin, are not recommended. Simply cooling the surface of the skin will not bring down your child’s body temperature. Tepid water baths of 85–90° F (29.4–32.2° C) may help bring down body temperature, but usually by only one or two degrees.

Piles of blankets to keep a child warm during feverish chills can negatively affect the body’s natural efforts to regulate temperature. Bundling infants less than 3 months old can actually increase body temperature. Light clothing and blankets are sufficient.

Keeping your child hydrated is critical during a fever or other illness. Bland foods and fluids are best. If your child has not wet a diaper or urinated in eight hours, has dry lips, and produces no tears when crying, this is a sign of dehydration and reason to seek medical attention.

What are febrile seizures?

While scary for parents, febrile seizures are rarely dangerous to children. These seizures are induced by a rapid rise in body temperature—not an exceptionally high body temperature. Most febrile seizures occur when a fever begins and, as such, there is no warning.

Febrile seizure affect 2–4% of all children under the age of 5, with children most susceptible at age 2. The vast majority are brief, lasting a matter of seconds or a few minutes. A child having a febrile seizure may present with classic jerking movements, or may simply appear to pass out.

If your child has a febrile seizure, slowly lower them onto their side. Do not put anything in their mouth or hold them during the seizure. The greatest risk from febrile seizures is the potential to fall or choke on food, saliva, or another object.

Seek immediate medical attention following a febrile seizure. It is unnecessary to call 911 unless a seizure continues beyond 15 minutes, or if the child does not recover quickly following a short seizure. Even febrile seizures lasting more than 15 minutes typically have good outcomes. Hospitalization is necessary only for children under 2 months of age, those with a potentially serious illness, or those with prolonged seizures.

Simple febrile seizures lasting less than 30 minutes do not increase the risk for unrelated epileptic seizures and do not cause brain damage or developmental delay. They are more common in children who have a family history of febrile seizures. Children who have had one febrile seizure are more likely to have another.

Your pediatrician and the pediatric urgent care professionals at KidMed are always available to help determine the cause of your child’s fever. Be wary of online information unless the source is trusted, like the American Academy of Pediatrics’ healthychildren.org.


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