Poisonous smog from peat bog fires has shrouded Moscow, prompting health officials to urge residents to lock themselves indoors as the city swelters in a record heat wave.

While Moscow routinely has hot summers, this year has been a record—breaker, with daily maximums around 35 degrees Celsius (95 Farenheit) for two weeks. The all—time high was set on Monday, 37.5 degrees (99.5).

The heat has exacerbated forest fires and peat bog fires east of Moscow, sending plumes of smog over the city.

Health officials have said the concentration of smog is so high that breathing it in for sustained periods was akin to smoking cigarettes, according to Russian news agencies.

The heat, which is relatively mild for the United States but alien to Northern Europe, is expected to ease in the coming days.

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