37 Microenvironments
| Code | Microenvironment | Code | Microenvironment | ||
| General | Specific | General | Specific | ||
| 1 | In vehicle | Car | 19 | Indoors | Restaurant |
| 2 | In vehicle | Bus | 20 | Indoors | Manufacturing facility |
| 3 | In vehicle | Truck | 21 | Indoors | School |
| 4 | In vehicle | Other | 22 | Indoors | Church |
| 5 | Indoors | Public garage | 23 | Indoors | Shopping mall |
| 6 | Outdoors | Parking lot/ garage | 24 | Indoors | Auditorium |
| 7 | Outdoors | Near road | 25 | Indoors | Health care facility |
| 8 | Outdoors | Motorcycle | 26 | Indoors | Other public building |
| 9 | Indoors | Service station | 27 | Indoors | Other location |
| 10 | Outdoors | Service station | 28 | Indoors | Not specified |
| 11 | Indoors | Residential garage | 29 | Outdoors | Construction site |
| 12 | Indoors | Other repair shop | 30 | Outdoors | Residential grounds |
| 13 | Indoors | Residence - no CO source | 31 | Outdoors | School grounds |
| 14 | Indoors | Residence - gas stove | 32 | Outdoors | Sports arena |
| 15 | Indoors | Residence - attached garage | 33 | Outdoors | Park/golf course |
| 16 | Indoors | Residential - stove and garage | 34 | Outdoors | Other location |
| 17 | Indoors | Office | 35 | Outdoors | Not specified |
| 18 | Indoors | Store | 36 | In vehicle | Train/subway |
| 37 | In vehicle | Airplane |
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