Spending on infrastructure provides jobs and is an investment in the future.

The hope is that the bridge may save the city from losing all relevance in manufacturing and trade-related services. Integration with China seems a better fate than total eclipse.

But there is one mystery here - why has Hong Kong got so noticeably dirtier in the past few years?

Why would a city as rich as this, looking for practical ways to spend its surplus cash, not invest more in the rich-world privilege of cleaner air?

One has to wonder if the answer lies with that bridge.

While fostering programmes to reduce regional pollution in the Pearl River Delta, Hong Kong has been appallingly slow about jumping on domestic sources of smog.

Heavily polluting diesel trucks, for example, are a key cause of roadside pollution in Hong Kong, but they are only slowly being phased out.

Surely the cost of a more accelerated phase-out would be a drop in the bucket compared with the HK$80 billion being spent on constructing an open-sea bridge and tunnel extravaganza - with Hong Kong pick- ing up the lion's share of the cost.

But then, imagine if environmental standards between Hong Kong and the rest of the Pearl River Delta were so starkly contrasting that residents in Hong Kong recoiled in horror as stinky mainland vehicles and smoky tourist buses trundled across the 40-kilometre, six-lane bridge.

Or imagine if Hong Kong's vehicular emissions standards were so tough that the environmental costs would be too prohibitive for regional logistics firms - as it is, we already will have to convince them to pay a bridge toll to use Hong Kong services instead of the many alternative choices on the mainland.

Or imagine if Hong Kong had ever had clean-air standards in line with those of Organisation for Economic Cooperation and Development members - if so, the bridge would have been a non-starter, as it would have likely failed any rigorous environmental impact assessments.

Whether consciously or subconsciously, it seems the powers-that-be decided that logistical convergence with the mainland was not possible without environmental convergence as well.

And that is exactly what is happening - in the past few years, Guangdong's air quality has been improving, while Hong Kong's has been largely getting worse.

A few years ago, the Civic Exchange - whose former chief executive, Christine Loh Kung-wai, is now Hong Kong's undersecretary for the environment - argued that foot-dragging on clean air measures might backfire: pollution might have been allowed to worsen to the point where other bridge-related infrastructure projects were scuppered.

Hong Kong hopes, for example, to build a third airport runway, in anticipation that lots of manufactured goods will be trundling across the bridge in future.

But with the air as thick as it already is, the third runway project may not pass the environmental assessment impact necessary to break ground.

If an expansion of Hong Kong's air cargo-handling capacity is blocked, this raises the odds that the bridge will indeed be at least a partial white elephant.

At the very least, it would lower potential returns on the bridge investment.

But perhaps the bigger question is whether Hong Kong took the wrong economic gamble altogether.

When policymakers first began considering the bridge idea more than a decade ago, the potential economic returns on clean air were less apparent.

Today, as smog in cities like Beijing and Shanghai reach crisis levels, intolerance for unpleasant air quality is growing.

Many highly skilled mainland employees - like those in the field of banking - may increasingly clamour to perform their duties in a city where they and their family can breathe the air.

Imagine if Hong Kong were that city. Instead, Singapore is.