根据输入的任务要求，给定一篇英文文章分配关键词。
任务要求：从给定文章中选择最相关的关键词，并指出它们在文章中的作用。

文章内容：
"A new study has found that exposure to nature, even in small doses, can have a significant impact on mental health. The study, which was conducted at the University of Michigan, found that just 20 minutes in the great outdoors can help to reduce levels of stress and anxiety. The researchers who conducted the study are now calling for greater efforts to promote the benefits of nature to both mental and physical health. According to the lead author of the study, Dr. Mary Carol Hunter, "Our study shows that even short-term exposure to nature can have remarkably positive effects on mental well-being. These findings suggest that accessible natural spaces within urban contexts may be a powerful resource for promoting mental health and combating the growing burden of mental health issues."
关键词: 自然, 心理健康, 压力, 焦虑, 城市环境, 心理健康问题

这篇文章阐述了自然环境对心理健康的重要性，并提出在城市环境中为促进心理和身体健康应更加努力地推广自然的好处。研究表明，即使在短时间内接触自然环境，也能显著地减轻压力和焦虑，与城市生活相比，自然空间可以成为促进心理健康和解决心理健康问题的强大资源。