## 1 Introduction  
Acetaldehyde (AA) or ethanal is a volatile compound belonging to the large family of aldehydes, a class of organic chemicals containing a formyl group. It is one of the most important aldehydes, occurring widely in nature and produced on a large scale in industry. It is also naturally present in many non-alcoholic beverages and in foods (e.g., bread, coffee, rip fruits, etc.) [1], as well as in the environment, since it originates from the metabolism of plants [2] besides being emitted directly into the atmosphere by combustion processes.  
AA is a metabolite of ethanal, which is oxidized in the liver to acetaldehyde by alcohol dehydrogenase (ADH) then to acetate by aldehyde dehydrogenase (ALDH) [3]. In October 2009, the International Agency for Research on Cancer (IARC) and World Health Organization concluded that AA from an alcoholic beverage and formed from ethanal endogenously is a Group 1 carcinogen to humans [4], whereas the molecule by itself is classified as possibly carcinogenic to humans (Group 2B) because of inadequate evidence on humans. Cell cultures and animal experiments have shown that AA is a mutagen and carcinogen, since it can cause point mutations and form covalent bonds with DNA [5; 6; 7; 8; 9].  
AA is also widely produced and used by industry, mainly as an intermediate in a large number of other chemicals and in the synthesis of flavor and fragrance acetals. It is also still voluntarily added in food manufacturing (e.g., milk products, baked goods, fruit juices, candy, desserts, and soft drinks) and is the designated as a \"generally recognized as safe\" (GRAS) synthetic flavoring substance by U.S. Food and Drug Administration (FDA). AA at low levels gives a pleasant fruity aroma, but at high concentrations it has a pungent irritating odor [10].  
AA is an unwanted food contaminant [11]. In alcoholic beverages, for example, it may be formed by yeast, acetic and lactic acid bacteria fermentation, or through auto-oxidation of ethanal and phenolic compounds [12]. Another source of AA contamination in food is polyethylene terephthalate (PET) which is widely used for food packaging and drink bottles [13]. PET can degrade under external factors such as sunlight and high temperature [14], and its main thermal degradation products are aldehydes, water, CO\({}_{2}\) and carboxyl end groups like carboxyl-terminated polyester chains but also terephthalatic acid and monoglycol ester [15; 16].  
Carbonyl compounds residues in PET polymers have been described for almost 40 years [17; 18; 19] in water. It is one of the contaminants most commonly released from PET bottles during heating or during any type of thermally induced degradation [20] and its presence has been monitored in bottled water samples collected worldwide, in Japan, US and in European countries [13] and, recently, in several state-wide surveys [21; 22]. All studies consistently report AA levels in PET bottled water, with higher levels of carbonated and higher storage time samples, demonstrating that this product is contaminated with residues migrating from the bottle and that contamination levels are regulated by multiple factors. In Italy, 14 billion liters of water were bottled in 2016 and the country has the largest European per-capita use of bottled water, with 206 L/year per capita; this is also the second largest in the world, after Mexico with 244 L/year [23].  
Here we present the results of a nation-wide survey of AA residues in bottled water. The aim was to quantify AA levels in bottled water products sold on the Italian national market, to assess whether their concentrations pose any risk for consumers' health, and to investigate which factors mainly influence the migration of AA into water in order to define approaches that could mitigate the consumers' health risk.