A department spokesman said that it needed to get a clear picture of the scale of the epidemic and what caused it.
At the same time, asthma experts have joined forces to try to make sense of the outbreak.
Dr Martyn Partridge, chest physician and chief medical adviser to the National Asthma Campaign, said that the hundreds who went to hospital at the height of the outbreak around 24 June were 'only the tip of the iceberg'.
He said: 'Many of these people were experiencing their first asthma attack and were mostly adults. They were not typical. This suggests to me that there was a much bigger number of people, including children who already suffer from asthma, who stayed at home or went to their GPs, or who treated themselves.'
He said that a full descriptive study of the event was now essential. 'Clearly the experts do not
all agree and there will be
disagreements if people start speculating without this information and begin to draw premature conclusions.'
The spate of cases began after thunderstorms in central and southern England on 24 June. Air pollution - especially from vehicle emissions - the storms, high pollen and fungal spore counts and high temperatures have all been implicated. What is not in dispute is that exceptionally high numbers of people with breathing difficulties were treated in casualty departments. Dr Partridge said two to eight cases a day would be normal in one unit in 24 hours.
'Hospitals were seeing 20, 40, 60, 80 and even, in one case, 100 cases in the same period.' In his own hospital, Whipps Cross, east London, 55 people attended the accident and emergency department in the evening and the following morning.
The number of people who died is not known.
The British Lung Foundation said the scale of the outbreak almost certainly meant pollution was a factor.
Dr John Moore-Gillon, an adviser to the foundation and consultant respiratory specialist at St Bartholomew's and Homerton hospitals in London, said: 'On the day that it happened pollution itself was not that terrible.
'But it's possible that air pollution had already primed people's lungs. It irritates them so that they are more sensitive.'
The Department of the Environment said it was 'not yet clear what role pollution played in it'. But the Government was funding research into the links between pollution and health and had set in motion consultations about ways of tackling pollution.