Air conditioning has had a tremendous impact on the United States, since Willis Carter developed an 'Apparatus For Treating Air' just 100 years ago, writes Ian Kilroy.

Imagine your forebears working the fields. Maybe it's a hot summer's day, there's hay to be saved. The rare Irish sun beats down burning their pale skin. In the heat of the outhouses the cows are bothered, waiting to be milked. Any idea of cool, humidity-controlled, sterilised milking parlours is a million miles away. It's 1902.

Meanwhile, in Brooklyn, a young engineer, Willis Haviland Carrier, has just invented air conditioning. Hot summer days will never be the same again.

In 1902 the Panama Canal had yet to be built, the Boer War had just ended and Irish tenant farmers were still under the yoke of the landlord, awaiting Gladstone's great Land Purchase Acts of 1903 and 1909. It was a very different world, just entering the decades that would collectively be known as "the American century".

Carrier was a graduate of Cornell University, with a masters degree in engineering. He had secured a job with the Buffalo Forge Company for a salary of $10 a week. His job: to design an apparatus to stabilise heat and humidity for the Sackett-Willhelms Lithographing and Publishing Company, whose paper kept changing size throughout the summer with the fluctuations in the temperature and moisture content in the air .

The company's problem was that as they added each new colour in printing it would be misaligned, as the paper size kept changing. A stable environment was needed to make four-colour printing possible.

Carrier came up with his "Apparatus For Treating Air". OK, the name wasn't exactly exciting, but his device lay the foundation for all later air conditioning systems. Carrier's invention would have a profound impact on the history of the world - and not just a reduction in BO problems on packed subway trains.

A recent article in the International Herald Tribune, dedicated to Carrier and his achievements, declared: "Air conditioning had a fundamental demographic and economic impact on the United States."

Citing a response to a question posed by Newsday to Richard Nathan, director of the Rockefeller Institute of Government at the State University of New York, the article suggested that the invention of air conditioning was as important to US history as the Civil Rights movement.

While a remark like that might raise the temperature of any admirer of Martin Luther King, you could also wonder if there is anything in it.

Well, it is true that cities like Houston, Texas, for example, could never have been built in the unbreathable air of the desert without Carrier's invention. Similarly, high-tech, precision instruments can now be manufactured in controlled environments in hot and sticky developing countries, where without air conditioning it would be impossible.

The uses of air conditioning in hospitals and intensive care units alone cannot be underestimated. The result of Carrier's invention is a world where manufacturing productivity soared, and where the phrase "chill out" came to have wide currency.

Anyone who has visited the US in summer will know how those cunning Americans have constructed an entire civilisation around Carrier's invention. And that civilisation, like so much in the American century, has a global reach. Walk into any designer shop in Rome from the scorching sun and you'll feel the gentle, artificial breeze of privilege brush your cheek. Thanks, Willis. In Ireland . . . well, maybe our need is not so great.

But are there any downsides to Carrier's legacy that the American press has overlooked in their optimistic celebration of the 100th anniversary of air conditioning? Well, the sudden chill of entering air conditioned buildings is rather shuddering. Just when you're accustomed to the heat, having left your pull-over at home, you enter the icicle air of a Paris chainstore and leave with a cold.

Then there are those "sick buildings" in New York, where the same air circulates from office to office, is artificially pumped from lung to lung - if one person in the building gets the flu, the entire building gets sick. The buildings aren't even designed with windows that open - you're forced to breath the same air as that guy who works in accounting 40 floors down, the guy with bad breath and yellow teeth.

To balance all the hot air in the US press it is important to remember the downsides of air conditioning.