I want to know the answer to the following question:
How many days of the year in 1991 was the air pollution a serious health risk?
All the background I'm provided with is that "Due to these initiatives and others, the air quality in Mexico City has begun to improve, with the air becoming cleaner since 1991, when the air quality was declared to be a public health risk for 355 days of the year.[citation needed]". Is that enough to answer the question?
Answer: yes