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The amount of liquid in a coffee cup is, however, limited. Furthermore, steam is a lot more dangerous due to the heat freed in condensation. Furthermore, there's a pretty large difference between 134 °C and 90. Finally, I don't think the mechanics of destroying infectious agents is really a great analogy for the mechanics of skin burns: the speed of reactions indeed increases non-linearly, yet the energy increases linearly. The energy in a coffee cup isn't nearly enough to damage the entire body; therefore the amount of damage is likely linked to the amount of energy released. Not that I'm volunteering for an experiment, admittedly. You mention the amount of time needed for serious burns to occur, yet that amount of time is huge compared to the time needed for water (or coffee) will flow away off or for people will to react. For her to remain exposed to the full load of coffee for around 12-15 seconds requires a pretty odd set of circumstances. For those burns to be large enough to be life-threatening normally requires a large surface area to be burnt, and that requires a large amount of coffee for a long time. (The 12-15 number is from wikipedia which says her lawyers produced evidence that this is the duration t 82 degrees celsius that "may produce third-degree burns"). Of course, there's the complicating factor here that she was quite old; at 79 she would not have been as resilient as younger person. And while that's a terrible shame, it's unreasonable to blame macdonalds for aging. Reading, she weighed just 47kg before her injury, which is low. I don't believe that a healthy person would have sustained her injuries. It sounds implausible that she could have managed to sustain those kind of injuries given the circumstances - unless some other factors played a role (such as frailty due to aging). It's terrible she suffered as she did, but I still don't see how that could have been reasonably foreseen by macdonalds (and therefore the verdict seems unreasonable). The point of the lawsuit isn't (just) about what happened to her, but critically also about the degree to which MacDonalds' was taking unreasonable risks. And as wikipedia points out, there have been many, many similar lawsuits, that apparently were not successful. (And note that coffee is still served at that temperature). |
As to water vs steam, steam has effectively a much higher heat capacity but far lower density. 90c water can cause 3rd degree burns in as little as 2 seconds but clothing generally extends the exposure duration significantly.
PS: In the end it was a low cost lawsuit which was settled for presumably less than 600k and well within the cost of doing business. They changed cup design not temperature, but that's about it. As far as their concerned the issue was not the actual cost but discouraging people from suing in the future as it's been 20 years without a repeat.