Iran issued an orange air-pollution alert on Monday for several major urban centers, with official monitoring showing unhealthy air even as the country observed a public holiday and many schools and universities were closed or moved online.

Forecasters said pollution could intensify through the end of the week in densely populated and industrial areas, warning that stagnant weather and temperature inversions could push air-quality readings in some places into the “very unhealthy” range.

The alert follows days of red readings in cities such as Tehran and Isfahan, highlighting a winter pattern in which vehicle exhaust, industrial output and heavy-fuel use combine with stagnant weather to drive smog spikes.

Tehran has recorded only six clean-air days so far this Iranian year (started on March 21), according to the capital’s Air Quality Control Company, and more than half of days have been unhealthy for sensitive groups – children, older adults, pregnant women and people with heart or lung disease.

With seniors making up about 8.4% of Tehran province’s population – roughly 1.2 million people – health experts warn that prolonged exposure during repeated pollution waves is elevating risks of respiratory and cardiovascular complications, adding to a crisis authorities have struggled to contain beyond temporary closures and driving restrictions.