One of my favourite things to do in New York is to go on long nature walks around the city. Sometimes I see a baby rat frolicking among the trash cans; sometimes I see a pigeon; sometimes I see a rat eating a pigeon. It’s rodent tooth v feral claw in the urban jungle.

Manhattan isn’t just richly populated with belligerent rats and pigeons. If you are lucky, you can also spot a mutant squirrel on your rambles. Thanks to massive inbreeding, New York is home to soot-black squirrels, which have a genetic condition called melanism. Inbreeding has also caused a group of brownish-orange squirrels to proliferate. “For squirrels, cinnamon is now the new black,” the New York Times proclaimed in 2001. OK, that’s enough squirrel facts. My point here is that while Manhattan is full of natural delights, they are of a limited variety. I love living in the city, but being surrounded by so much concrete and so many squirrels can occasionally drive me up the wall. I need to regularly escape into the countryside and see some trees for the sake of my mental health. And I am obviously far from the only person to feel like this. Humans were not built to live in densely populated cities, breathing in car fumes as we shuffle from one indoor place to another. The emergence of trends such as “forest bathing” shows that many of us are desperate for more trees in our lives.

Access to green space is important for both mental and physical health. There is plenty of research that backs that up, but it is also just common sense, something most of us viscerally feel. Increasingly, however, access to nature is becoming a luxury: growing inequality has resulted in a “green gap”. A recent study by researchers at the University of British Columbia, for example, found that access to parks and green space in American metropolitan areas correlates with class, education and race. The whiter and richer you are, the more likely you are to have access to a few trees.

It is a similar story in the UK. According to a 2013 report by the National Children’s Bureau, for example, the least deprived children in Britain are “nine times more likely than those living in the most deprived areas to have access to green space, places to play and to live in environments with better air quality”.

Not having access to parks and other greenery means a higher likelihood of developing asthma or other respiratory diseases. It means a higher likelihood of anxiety and depression. It means fewer opportunities to go out for a jog, fewer opportunities to sit quietly for a moment and simply smell the flowers. As urban populations continue to grow, it is crucial that we reduce the green gap between the rich and the poor. Access to fresh air and green grass should not be a luxury only the rich can afford.

• This article was amended on 1 May 2019 to prevent repetition.