This functions converts a pollutant value in its original units into one of the 5 categories used by the Mexican government to communicate to the public how polluted the air currently is and its health risks.
a numeric vector of values to convert to index
type of pollutant. A vector of one or more of the following options:
the IMECA value of the concentration indexed into 5 categories
Good: 0-50 minimal health risk
Regular: 51-100 moderate health effects
Bad: 101-150 sensitive groups may suffer adverse heatlh effects
Very Bad: 151-200 everyone can experience negative health effects
Extremely Bad: > 200 serious health issues
convert_to_index(c(12.1, 215, 355), c("PM25", "PM10", "PM10"))#>  "REGULAR" "MUY MALA" "EXTREMADAMENTE MALA"