Sampling location | Region | Study counta | Population (million)b | Reference |
---|---|---|---|---|
Austria | Europe | 3 | 8.5 | |
Canada | North America | 2 | 35.5 | |
Finland | Europe | 2 | 5.5 | |
France | Europe | 2 | 66.2 | |
Germany | Europe | 1 | 80.9 | [10] |
Hong Kong | Asia | 2 | 7.2 | |
Internationalc | – | 5 | – | |
Singapore | Asia | 1 | 5.5 | [18] |
South Korea | Asia | 2 | 50.4 | |
Spain | Europe | 1 | 46.4 | [21] |
Taiwanb | Asia | 1 | 23.4 | [22] |
United States | North America | 50 | 318.9 | |
Others | – | 0 | 6,612.0 |