The latest in environmental tourism?

It would take the French.

A tethered gas balloon in Paris's 15th arrondissement has been taking tourists up for aerial views of the city since 1999. Now, it has been coupled to an air quality monitoring and display system, to give real time information on air quality in the city and at major traffic junctions.

Sensors will monitor levels of nitrogen dioxide, ozone, and particulate matter. And, if we understand this correctly, 'Ballon Air de Paris' will change colour depending on air quality: red for highly polluted air, through to yellow for 'mediocre', and green for very clean.

Perfect, you'd have thought, for keeping an eye on air quality in Beijing in the run up to the Olympics? No, you're right. It would never take off!

How many people are there in the world?

6,681,394,380 . . . and counting . . . rather fast.

Today, July 11, is World Population Day, inaugurated in 1988 by the United Nations to mark the day when the world's population hit five billion, July 11, 1987.

The counter is at www.worldometers.info, where we learn that the absolute growth in population today (at the time of writing) is 135,468. Watching all the numbers spin around can be mesmerising. And worrying.

Some of the counters clock up slowly -- the number of women who died in childbirth, for instance, although that depressing total is already over 330,000. Reassuring, on the other hand, if we can believe the statistics, that world expenditure on education is nearly twice military expenditure.

Other eye-catching comparisons: as a committed city cyclist, it's nice to see that twice as many bicycles have been produced as cars this year, although I'm intrigued to see that there are three times as many computers as bicycles.

And can there really be nearly half a million new book titles so far this year? (Enough to humble any author who will soon be adding to the pile!)