Excessive heat warnings have been issued for portions of 21 states, with temperatures expected to reach 115 degrees in some places. Thermometers in Pierre, South Dakota registered 105 degrees on Wednesday afternoon.

The heat wave already striking at the nation’s midsection will head east, spreading toward the Northeast and mid-Atlantic tomorrow and into the weekend.

Hot air trapped underneath a high-pressure system is responsible for the “heat dome” which is causing the intensely hot and humid conditions.

Who is affected?

Heat alerts have been issued for states from Louisiana to Minnesota, parts of Michigan, Indiana, northwest Ohio, Nebraska, the Dakotas and Illinois and Kentucky and all of Iowa -- more than 200 million Americans in all.

Temperatures in the Southeast are forecast to range from the mid-90s to 100 degrees.

Outdoor workers at risk

Among those for whom excessive heat is especially dangerous: outdoor workers. OSHA is reminding employers that workers must be monitored for the signs of heat exhaustion and heat stroke.

In 2014 alone, 2,630 workers suffered from heat illness and 18 died from heat stroke and related causes on the job.

To protect workers from excessive heat, employers should:

• Provide workers with water, rest and shade.
• Allow new or returning workers to gradually increase workloads and take more frequent breaks as they acclimatize, or build a tolerance for working in the heat.
• Plan for emergencies and train workers on prevention.
• Monitor workers for signs of illness.