The US has come under a “Heat Wave Warning” from the National Weather Service saying “Dangerous Levels” could be reached this week affecting 100 million Americans across several states from Texas, Louisiana to Kansas and Missouri.

The summer heat is getting intense, so hot that forecasters put nearly 100 million Americans under excessive heat warnings or advisories this week. The National Weather Service says the “dangerous heat” is forecast to impact a large swath of the country, including states ranging from Texas and Louisiana to Kansas and Missouri.

Heat wave is sweeping across Europe and London alone experienced the seasons highest off 40 degrees Celsius on a single day sending Londoners to the nearest air conditioners and fan shops to buy them to cool themselves down, reports from London said.

Politicians are blaming global warming as the reason for climate change leading to unusual heat waves in countries which have not experienced such temperatures.

A number of states were also placed under a heat advisory with most areas forecast to reach at least the high 90s (Fahrenheit) and other areas, including Phantom Ranch, Ariz., on track to reach a whopping 114 degrees. Amarillo, Texas, is similarly expected to reach a high of 113 degrees and Shreveport, Louisiana, will soar to 108 degrees, according to a report in the USA today quoting NWS sources.

Arkansas, Louisiana, Oklahoma, Tennessee and Texas are expected to bear the brunt of the highest temperatures, forecasters say.

“This warning is reserved for only the hottest days of the year and is issued when temperatures are expected to rise to dangerous levels,” the National Weather Service wrote in an excessive heat warning.

The Weather Service places areas under an excessive heat warning when the maximum heat index temperature is expected to be 105 degrees or higher for at least two days and temperatures won’t dip below 75 degrees. A heat advisory is posted for an area when temperatures are expected to reach 100 degrees for at least two days.

The “extreme heat” will continue through the week and expand across the Northeast on Wednesday, according to the Weather Service. A majority of the excessive heat warnings mostly cover the southern Plains and lower Mississippi River Valley.

Several heat advisories have also been placed in portions of the Northeast, ranging from Philadelphia to Boston. In those areas, the heat index is expected to approach 100 degrees.

Heat index is how hot it really feels when humidity is factored in with the actual air temperature, according to the Weather Service. For example, if the relative humidity reaches 70 per cent and the temperature is 90 degrees, the heat index will be 105.

In addition to high temperatures, several states are also seeing heightened fire danger. The Weather Service placed portions of Louisiana, Nebraska, Oklahoma and Texas under red flag warnings. These warnings, USA today says, mean weather conditions, such as high heat, low humidity and strong winds, along with dry vegetation are providing the perfect mix for wildfires that could rapidly grow out of control.

Almost 90 large fires are actively burning across 13 states, 58 in Alaska alone. Over 3 million acres are covered in US large fires, as of Tuesday. The largest active wildfire in Alaska is up to 865,620 acres wide in McGrath. It is 56 per cent contained and expected to be fully contained by August 1.

The Weather Service is advising people to take extra care when outside and do things like drinking plenty of fluids, staying in an air-conditioned room and getting out of the sun. “Take extra precautions if you work or spend time outside. When possible, reschedule strenuous activities to early morning or evening. Know the signs and symptoms of heat exhaustion and heat stroke,” the National Weather Service wrote in a heat advisory warning.

As temperatures rise, it is important to know the symptoms for heat exhaustion and heat stroke. Heat exhaustion symptoms include dizziness, profuse sweating, pale skin, nausea, weak pulse and muscle cramps. Someone experiencing a heat stroke may have a pulsating headache, dry skin, nausea and even lose consciousness. If someone is experiencing these symptoms, they should immediately seek medical assistance and try to cool down.