Don't discount the impact of reduction of air pollution on all causes of mortality. It's not often discussed, but every so often someone makes the connection between small particulates and heart attacks, etc.
Pretty sure any impact from that would be a very long-term thing. A lifetime without particulates might have an effect; a month seems pretty far-fetched.
"In 2004, the American Heart Association issued a scientific statement concluding that exposure to air pollution contributes to cardiovascular illness and mortality. A 2010 update elaborated on those risks.
Short-term exposure can increase the risk of heart attack, stroke, arrhythmias and heart failure in susceptible people, such as the elderly or those with pre-existing medical conditions, according to the statement."
Seems like a pretty subtle and non-acute thing. Maybe even analogous to smoking: something that gradually erodes your health, not something that suddenly kills you.
I would tend to think there is both long term damage and chance of increased immediate death due to things like asthma and heart attack triggered by polluted air.