Millions of Americans are being urged to remain indoors as dangerous air quality alerts have been issued across at least 11 states, including New Jersey, New York, Connecticut, and parts of Southern California. The National Weather Service (NWS) has reported that these alerts are primarily due to high levels of ground-level ozone in the Northeast and wildfire smoke in California. Major urban areas such as New York City, Philadelphia, and Los Angeles are particularly affected. Health officials recommend that residents limit outdoor activities, especially during peak afternoon hours, and keep windows closed while using air conditioning or air purifiers. The NWS attributes the poor air quality in the Northeast to ground-level ozone, which forms from vehicle and industrial emissions reacting with sunlight, exacerbated by high temperatures and stagnant air. In Southern California, active wildfires are contributing to the hazardous conditions, with smoke potentially reaching unhealthy levels. Air pollution poses significant health risks, especially for vulnerable populations such as children, older adults, and individuals with pre-existing respiratory conditions.