These are the 10 best destinations in the world for an eco-friendly vacation

Kayak's new City Index for Mindful Travelers ranked 167 cities across the world that offer sustainable — or eco-friendly — options for travel.

The study looked at 28 factors and grouped them into four categories "that reflect the main areas of consideration when planning a city trip," according to the report:

Each city ranking was determined based on searches from Jan. 1, 2021 to March 1, 2022, for the travel period of Jan. 1, 2022 to Dec. 31, 2022.

Rotterdam and Amsterdam, both in the Netherlands, took the No. 1 and No. 2 spots on the list.

According to the report, Rotterdam topped the list because it's not "over-touristed" and you can fly there in the off-season month of January for 59% less on average than the high-season month. 

The Netherlands city is the second-largest in the country and Europe's largest seaport. Rotterdam is known for its riverside setting and modern architecture.

Amsterdam ranked No. 2 thanks in part to its lower-emitting transport options, such as ferries and trains. The city, known as "the Venice of the North," is now a designated UNESCO World Heritage site, thanks to its iconic canal district.

The channels' design is considered an example of town planning that influenced the world for centuries, according to UNESCO.

In addition to its historic canals, the capital city's main attractions include the Van Gogh Museum, the Anne Frank House, and a robust nightlife scene.