The Solomons had a rating of 5.0, ahead of New Zealand (5.2), New Brunei Darussalam (5.4) and Australia (5.8).
Despite having the cleanest air, according to the report the country still falls behind on other development indicators, particularly in areas such as life expectancy, improved access to proper sanitation and rates of cancer.
The average life expectancy of Solomon Islanders is 69.2 years, below the global average of 71.4 years.
Out of every 1000 babies born, WHO said its data showed 114 would die – though it was better than the global average of 212 deaths per 1000 babies born.
The Hapi Isles has 22.1 health professionals for every 1000 people, well below the global average of 45.6 per 1000 people.
WHO estimates 26.4 per cent of Solomon Islanders aged between 30 and 70 will die from either cancer, diabetes, cardiovascular disease or respiratory disease, above the global average of 18.8 per cent.