The Housing Authority may have to review the penalty scheme it uses to punish dirty tenants if a second challenge to an eviction under the system succeeds. No one has been evicted yet under the two-year-old scheme. A tenant ordered to leave after accumulating 17 points for littering and spitting lodged an appeal with the authority yesterday. Under the scheme, introduced in the aftermath of the Sars outbreak, tenants are given five demerit points for littering and seven for spitting or throwing objects from a height. Tenants face eviction if they get 16 points within two years. A family whose tenancy was terminated last October for littering twice and throwing an object from a height had the decision overturned two months later by the authority's appeal panel. Two other tenants have appeals pending. One faces eviction for keeping a large amount of waste in the flat and the other for littering and spitting. An authority member, City University Professor Anthony Cheung Bing-leung, said it was too early to conclude that the marking scheme was not working, although he said it might have to be changed. 'If the tenant successfully challenges the authority's decision this time, we will have to review the marking scheme. Otherwise it will not act as a deterrent to tenants.' The outcome of the appeal will be known in about 14 working days.