Chemical analysis conducted by iFixit and HealthyStuff.org has tracked a decline in hazardous and toxic chemicals in smartphones over the past few years, with new devices like the iPhone 5 representing some of the safest and most environmental handsets yet, The Next Web reports.

The study took apart 36 mobile phone models and studied them using X-ray fluorescence spectrometry to check their chemical composition.

“The researchers then rated and ranked the phones on a scale of 0 – 5, lowest being best, in three ways: by chemical (for each of 12 commonly found hazardous chemicals, such as bromine, mercury, and lead), by component (case, screen, solder, circuit board, etc.), and overall,” iFixit noted.

Six phones got a low enough score to be considered of “low concern”: the Motorola Citrus (2.56), iPhone 4S (2.69), LG Remarq (2.69), Samsung Captivate (2.71), iPhone 5 (2.75) and Samsung Evergreen (2.81).

24 of the phones were rated as “medium concern”, and six handsets were marked as “high concern.” The six highest scores all came from devices made before 2010 and included the original iPhone, which received the maximum score of 5.00, the Palm m125 (4.58), the Motorola Renew (4.56), the Nokia N95 (4.50), the BlackBerry Storm 9530 (4.41) and the Palm Treo 750 (4.26).

Although the iPhone 4S scored slightly better than the iPhone 5, the study did show steady improvement from Apple over the years.

“The iPhone has undergone a steady, gradual toxic chemical improvement in the last five years: the iPhone 2G received the worst overall score of all ranked phones, but the iPhone 4 and 5 are now among the top ten percent of phones,” the report read.