I am very sorry to learn about your situation.
Unfortunately, there is no basis to overturn your purchase simply because you have buyer's remorse.
There is a "3 day cooling off" period that applies to a very select few contracts (see: http://www.consumer.ftc.gov/articles/0176-buyers-remorse-when-ftcs-cooling-rule-may-help), but they do not apply to your situation.
The best I could offer you as a suggestion would be to open up all of the documents you have related to your purchase - lay them out and carefully read each clause. Check to see if there is any provision in the contract that allows you to cancel the sale. (Odds are that there is not, but without a careful review of the contract you will not know for certain).
Regarding the smells in the unit - it appears that while you told everyone there you were sensitive, they also advised you that there was nothing they could do to assure you of the character of the smells: a used unit carries a risk of mold, while a new unit carries the risk of chemical smells associated with construction. They did not conceal or defraud you of the character of the units.
Regarding the safety mechanism of the C02 alarm, there is no basis to revoke a sale because the item conforms to common safety standards.
You can always try to negotiate with them, they may be willing to accept back the unit in exchange for you paying a "restocking fee" that will cover the cost of delivery plus any depreciation (the value they will have to mark down the unit to resell it to a subsequent purchaser), but unless you are able to find something in your contract, they do not have any obligation to cancel the sale and return your money based on what you have posted here - you can always speak to a local attorney and discuss your matter in more detail (they can provide you with a formal legal opinion and specific advice - both of which are outside of the "general legal information" scope of this forum).