Northern Utah is in the throes of its worst inversion so far this winter, with concentrations of fine particulate matter, or PM2.5, reaching unhealthy levels in Salt Lake, Weber, Davis and Cache counties.

An inversion has been building since Tuesday, trapping pollution in the valleys and obscuring mountain views all along the Wasatch Front. Officials imposed mandatory measures to reduce emissions, such as restrictions on wood burning, and encouraged residents to reduce automobile travel.

By Friday afternoon, PM2.5 measurements were double the federal limit in Salt Lake City and climbing farther into the red. Conditions were even worse in the Cache Valley, where measurements reached 85 micrograms per cubic meter.

Exacerbating Utah’s poor air quality is the snow blanketing the valley floors. Sunlight reflecting off the snow cover accelerates the formation of minute particles in the atmosphere that get into people’s lungs and lead to various health problems.

Those with underlying health concerns are encouraged to remain indoors through Sunday when the weather is forecast to change.