Atom supports Mac OS X 10.7+ (64bit Intel CPU only) and Windows 7 - 10 (32bit & 64bit Intel-compatible CPU with SSE4 instruction set). 

32 Bit is an extremely outdated operating mode, and many modern daws have dropped 32 bit support entirely (like Ableton for Ableton 10), so we have done the same on OSX. 

There is a 32 bit port that can be used potentially on Windows however. Such a port is not possible on Mac because Apple has dropped the ability for developers to make 32 bit products:
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​"Additionally, starting with OS X Lion, only 64-bit Intel Macs are supported, so software that specifically depends on new features in OS X 10.7 or newer will only run on 64-bit processors and therefore does not require 32-bit/64-bit fat binaries."

Thus any operating system on Mac older than OS X Lion cannot use Atom unfortunately.
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In terms of DAWs / host softwares Atom is supported by Pro Tools 10.3.10 or higher, along with any VST/VST3/AU-compatible host running on a supported operating system.
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