Hundreds of thousands of Americans have been urged to stay indoors and shut their windows as a major air quality warning alert hits the Upper Midwest. Roughly 300,000 people in Minnesota , North Dakota and South Dakota have been engulfed in a plume of hazardous and unhealthy air fueled by dust storms sweeping across the northern US. The US Environmental Protection Agency (EPA) declared the air over Watertown, South Dakota - home to nearly 24,000 people - to be 'hazardous' to human health Thursday afternoon. According to the World Air Quality Index project, a nonprofit providing real-time air pollution data for hundreds of countries, the air quality index (AQI) in Watertown reached 439 at 1pm ET.

When outdoor air quality reaches the 300 to 500 hazardous zone, the EPA warns that everyone should avoid all physical activity outside. Meanwhile, another plume of 'unhealthy' air was detected over Fargo, North Dakota, a city of more than 130,000 people right on the border with Minnesota. The main pollutant in the dust storms has been PM10, a type of fine particulate matter made up of tiny solid particles or liquid droplets floating in the air that are less than 10 micrometers in diameter, thinner than a human hair. This form of air pollution is noticeably larger than PM2.5, which is typically composed of toxins or heavy metals produced by factories and car exhaust, but it can still damage the lungs, worsen respiratory issues such as asthma and contribute to heart attacks and strokes that cause premature death.

The air quality alerts come as a massive Red Flag Warning has been issued across at least seven states, including Colorado, Iowa, Nebraska and Wisconsin. A Red Flag Warning means that weather conditions have reached a critical tipping point for sparking major fires within the next 12 to 24 hours. This includes extremely low humidity, making the region overly dry and creating fuel for wildfires with dead vegetation, and strong winds over 20 mph. However, local forecasts have warned that this week's winds are expected to reach between 40 and 60 mph. The winds have picked up loose, dry topsoil, mainly from farms and open prairie land, and have tossed them into the air, making it difficult to breathe for anyone outdoors.

The National Weather Service warned in a statement : 'Particle pollution can get deep into the lungs and cause serious health problems such as asthma attacks, heart and lung disease symptoms, and increased risk of lung infections.' 'Everyone can be affected, but sensitive groups such as people with lung or heart disease, older adults, people who are pregnant, children, and those who spend a lot of time outdoors are at greater risk.' Health officials also urged everyone within these zones to keep windows and doors closed to limit outdoor pollution from leaking inside. Using air conditioners or air purifiers can help as well.

The air quality alert in Minnesota is scheduled to remain in effect until 10pm ET on Friday. The conditions are also expected to become extremely dangerous for drivers, with thick dust causing visibility to deteriorate heading into the weekend. Officials in northern Minnesota urged drivers to be ready to stop suddenly when visibility nears zero. If drivers encounter dust or sand blowing through roadways or see an approaching storm, they should pull over as far as possible and put the vehicle in park. Forecasters with KAXE in Minnesota added: 'Turn the lights all the way off and keep your foot off the brake pedal. Remember, "Pull Aside, Stay Alive."'