Testing pollution levels
Two pictures, each taken this year from a block of flats in the centre of Beijing.
The one on the left is what Beijing hopes you'll see when the Olympics begin in a month's time. The one on the right is what you'll see if its pollution control plans fail.
When it bid for the Games in 2001, Beijing promised the International Olympic Commission that its air would be normal: "Air and water quality is expected to be within World Health Organization standards at all venues, and in Beijing generally." (Report of the IOC Evaluation Commission for the Games of the XXIX Olympiad.)
Compare that promise with the warning issued by IOC president Jacques Rogge in August 2007: if the air is still bad during the Olympics, some endurance events might have to be postponed or delayed.
So, Beijing's air will be fundamental to the success or failure of this city's Games.
How serious is the problem? We've carried out our own investigation into Beijing's air quality. Let's break it down.
The location: we tested every day for seven days at a fixed point along Yonghegong avenue in the centre of Beijing. This is part of the route of the individual road race in the men's cycling - an important endurance event. We used a hand-held machine that has a 20% or so margin of error.
The pollutant: We tested for airborne particles known as PM10 - a source of much of this city's pollution. These particles come from traffic, factories, building sites, and occasional dust storms.
The standard: the World Health Organization's air quality guideline for PM10 is a maximum of 50 micrograms/cubic metre (averaged over a 24-hour mean). For more polluted cities, the WHO has set several interim targets, the most generous of which is a maximum of 150 micrograms/cubic metre.
01/07/08 : 121 micrograms/cubic metre
02/07/08 : 172
03/07/08 : 122
04/07/08 : 351
05/07/08 : 112
06/07/08 : 27
07/07/08 : 242
The conclusion: One day lived up to the WHO's air quality guidelines.
Three days lived up to the WHO's more generous interim targets.
Three days were so polluted that they were outside even the WHO's most generous targets. The most polluted day - 4 July - was seven times worse than the WHO's air quality guidelines.
Comparison with London: It's worth comparing these readings with London - the city that hosts the Games after Beijing. PM10 readings taken during the last week by the London Air Quality Network in sites in East London (the site of the 2012 Games) were all within the WHO's air quality guidelines.
Our quick investigation shows that, with just one month to go, Beijing faces a real test if it's to live up to its Olympic promise of clean air.
The city's officials realise this. So they've decided to impose a series of emergency air quality measures. On 20 July half of Beijing's cars will come off the streets, building sites will stop work, and a number of factories will shut down. Even outdoor spray painting will be banned.
This gives 19 days for the dust and the airborne particles to settle before the first event is held.
If these measures don't work, the city says that it's prepared to take what it calls "more stringent measures" during the Olympics. But it doesn't say what these include.
"If I have to give it a simple answer then I'll say that we are confident we will fulfil our promise," says Du Shaozhong from the Beijing Environmental Protection Bureau. "The World Health Organization has set out perameters. So according to these guidelines we can achieve the target. This is not a problem."
The IOC has told the BBC that it's watching to see what impact Beijing's plans will have.
A month from the Games, the task is clear: in order for the Beijing Games to be a success, this city's last-minute emergency air quality measures will have to work.
We're going to keep on testing the air in the run-up to the Games - to see how Beijing gets on with its clean-air promise.
Every day from now till the games begin we'll take a PM10 reading in central Beijing. We'll also take pictures of the city's skyline so that you can see for yourself what the air's like. You can follow it all here on the site.
Please send in your photos as well - we'll try to feature as many as we can.