What type of research did Dr. Ole Raaschou-Nielsen conduct regarding air pollution?

Dr. Ole Raaschou-Nielsen is known for his significant contributions to research on air pollution, particularly in how it affects public health. The correct answer highlights that he looked at data from various European countries, which reflects the comprehensive and comparative nature of his study. By analyzing data across different regions, he was able to draw more general conclusions about the effects of air pollution on health outcomes. This approach helps in understanding the broader implications of air quality and its impact on populations with varying environmental exposures and health systems. This comparative analysis enhances the validity and applicability of the findings across different contexts, making it a cornerstone of his research.

In contrast, analyzing data from just five studies, conducting laboratory experiments, or surveying public health officials would not encapsulate the extensive scope and relevance of Raaschou-Nielsen's work as well as the examination of diverse datasets from multiple countries. Each of those alternatives may focus on narrower aspects, but they do not reflect the broad, integrative perspective that characterizes his impactful research.