How is OZONE emitted from portable air purifiers? Is it from the IONIZER that come on some?

We own several pet birds and have heard ozone is bad for them. We heard ozone is sometimes emitted from these home air purifiers that we must use. Ours have a swtich to either use or NOT USE the ionizer. Can anyone tell us how ozone is generated and sent off into our air from Air Purifiers?