The 3 hourly PSI Reading is completely useless. The 24 hour PSI Reading is even worst.
Let me explain why it is useless as a practical tool to know what one should do, especially those who are vulnerable because of underlying health conditions, elderly and the very young.
If the actual hourly PSI levels for 6 consecutive hours is 50, 100, 150, 200, 250 & 300 respectively, the 3 hourly PSI level will read as follows;
1) 100 at 3rd hour (when real PSI is 150)
2) 150 at 4th hour (when the real PSI is 200)
3) 200 at 5th hour (when the real PSI is 250)
4) 250 at 6th hour (when the real PSI is 300) etc.
So one will mistaken that it is just at 'Very Unhealthy' level at the 6th hour when it is already breathing air that is already at 'Hazardous' levels.
What is the purpose of the 3 hourly PSI and 24 hour PSI readings? Your guess is as good as any.