I have 70 % lung capacity but it drops to 50 - 53% when im ill. what type of exercise can i do to work it up to 80% or am i being unrealistic.?

Rehab: Pulmonary rehab includes high exercise level practicing. One of the most important things. Needs follow up with a lung specialist!

Mostly unrealistic: If your lung function falls with illness it implies airway inflammation from the illness with decreased flow. Other considerations include neuromuscular disease worsening with illness. Genetics determine our total lung capacity and the number of alveoli in a lung is predetermined according to crapo and no amount of exercise can change that. Once you lose alveoli there is no recovery.

See below: Don't know how old you are and what kind of lung disease you have, but in general, if you have asthma, you may be able to work it up; if you have COPD or pulmonary fibrosis, for example, you won't be able to work up the number but if you get into a pulmonary rehabilitation program, you may feel a whole lot better with the same number for your lung capacity.