New Delhi, March 21 -- Whenever people ask me what I miss about San Francisco, a city I lived in for many years, I have some ready answers.

I miss the sushi at the Japanese restaurant I used to go to, the fish gleaming and fresh, the wasabi bracingly stinging. I miss the late-night al pastor quesadilla at the local taqueria, the smell of barbecued meat and cheese making my mouth water as it sizzles on the hot pan. I miss dim-sum coming by in little trolleys at a Chinese restaurant-trays of char siu buns, crispy yet tender pork belly with Chinese mustard on the side and sticky rice wrapped in leaves.

But this year when I arrived in San Francisco from Kolkata I realised what I had missed most of all was fresh air.

San Francisco AQI was 3...