Delhi ranked world's most polluted major city on eve of Diwali, Swiss group says

1. What happened

   According to a Reuters headline, a Swiss group reported that Delhi was the world's most polluted major city on the eve of Diwali. The source excerpt does not name the group or give its air-quality figures.

2. When it started

   The report refers to the eve of Diwali, but the exact date is not given in the material provided.

3. How it spread

   The claim comes from a Swiss group whose methodology is not described in the available source. Reuters is the reporting outlet named in the headline.

4. Why it matters

   Diwali typically brings heavy fireworks and smoke, and air pollution in Delhi is a recurring public health concern. The full article would be needed to explain the group's measurements and any official response.