为什么是转折？

Pulmonary effects of oxygen vary greatly between different species, probably because of different levels of provision of defences against ROS. Most strains of rat will not survive for much more than 3 days in 1 atm of oxygen. Monkeys generally survive oxygen breathing for about 2 weeks, and humans are probably even more resistant. Oxygen tolerance in humans has been investigated, but these studies are based on reduction in vital capacity, and so on, which is a very early stage of oxygen toxicity. There is an approximately inverse relationship between Po2 and duration of tolerable exposure. Thus 20 hours of 1 atm had a similar effect to 10 hours of 2 atm or 5 hours of 4 atm.

氧的肺部效应在不同物种间差异很大，可能是因为对ROS的防御水平不同。多数品种的大鼠在1个大气压的氧气中存活的时间不会超过3天。猴子一般能存活两周左右，而人类的抵抗力可能更强。已有对人类耐氧性的研究，但这些研究是基于肺活量减少等基础上进行的，肺活量减少是氧中毒极早期的阶段。P_O2与可耐受暴露的持续时间近似反比的关系。因此，暴露于1个大气压下20个小时与2个大气压下10个小时或4个大气压下5个小时的损害相似。

已有对人类耐氧性的研究，但这些研究是基于肺活量减少等基础上进行的，肺活量减少是氧中毒极早期的阶段。

为什么是转折？文章又没说不再继续研究下去